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Discussion Starter #1 (Edited)
I've been searching for a smartphone/tablet car mount lately and, since I didn't find anything appealing to me, I decided to make my own.
It had to be functional (obviously), seamlessly integrated in the dash and cheap. I never liked the look of suction, air vent or CD-slot mounts, so it came down to magnetic ones.
As far as I know, magnets have no adverse effects on modern phones or tablets as SD and SIM cards are not affected. Only the compass sensor (for those devices that still have it) could become potentially uncalibrated. After all, there are various commercially available magnetic accessories (mounts, folio cases) in existence. Do your research though and make an informed decision on your own. You must also keep in mind that magnets do affect pacemakers, credit and membership cards (at least those with the strip on the back), so exercise caution around those items.

When you shop for magnets, you should know that the pulling force of a magnet depends on its size, material and grade. I bought Neodymium (very strong rare earth material), N45 grade (the greater the number the stronger the magnet) ones. For my application, the preferred size was 15mm X 15mm X 3mm. Naturally, bigger dimensions result in stronger magnets. You can also stack two ore more magnets together and you will end up with approximately the pulling force of a single magnet of the combined size.
Moreover, the magnetic field of a magnetic dipole is approximately proportional to the inverse cube of the distance from the dipole. That means that the pulling force will diminish considerably if you separate the magnet from the surface you wish to "hold" even by mere millimetres. The following link of an online calculator might come in handy:
K&J Magnetics - Magnet Calculator
So, factor the weight of the device you wish to mount and the distance of the magnet from the metallic surface on the device into your calculations and choose accordingly.
In my case, the two magnets have a nominal pull force of 3.5kg each. More than enough for my 0.121kg phone and 0.337kg tablet, right? More on that later.

Anyway, I chose to modify two switch blanks of the dash to create candidate mounts. Each of them has a surface area of 42mm X 24mm (try to acquire a magnet that covers as much of that area as possible).
I used double-sided adhesive tape, a piece I cut from a mouse pad, some PVC insulating tape and two 15X15X3 magnets

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50698-switch-blank-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)
and came up with these two variations (eventually I opted for the glossy black one, although I wish I could find a rubbery black material for a more "professional looking", grippier surface):

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50706-magnetic-mount-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)

I could place it on one of the empty slots on the cigarette lighter trim piece but it would be unsafe to glance at the screen down there while driving.
Instead, I decided to replace one of the three switches below the climate controls with the mount. To gain access to those switches, the stereo has to be removed
(instructions on this task can be found here: http://www.rav4world.com/forums/84-4-1-accessories/78966-upgrading-stereo-mounting-kits-dash-frames-necessary.html#post834207):

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50714-dashboard-access-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)

By personal preference, I chose the middle spot to place the mount. Consequently, the position of the existing switches had to be rearranged.
I elected to move the rear window defogger switch to the left-most slot (the wiring loom is marginally long enough), the hazard lights switch to the right-most slot and the diff lock switch to a slot on the bottom switch panel, next to my "sliding gate remote control" switch
(more on that here: http://www.rav4world.com/forums/94-4-1-d-i-y-modifications/223449-oem-garage-door-opener-h0m3l-nk.html).
For the latter, I had to make a relocation harness. I could extend the wiring by cutting the factory loom of the switch and soldering extension wires but I like my mods to be completely reversible.
I bought a center diff lock switch connector (WITH ITS PIGTAIL) from a crashed Rav (#90980-10997), about 2 metres of wire, five "MDV 110(8)" male spade connectors and some heat shrink tubing

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50722-requirements-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)
and came up with this:

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50730-diy-loom-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)
If you use one-colour wire for your extensions (as I did), be sure to mark the terminals accordingly. The diff lock switch plug has 10 slots, only 5 of them though are used ("2","3","7","8" and "10" in the following picture):

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50738-centre-differential-lock-plug-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)
Since we are tampering with electrical circuits, it is good practice to disconnect the negative pole of the battery to avoid any problems. Detach the bottom switch panel and pop out a spare switch blank.
Connect the 5 male ends of the relocation harness to your factory diff lock plug and route the other end through the slot of the removed switch blank. Zip-tie wherever it makes sense.
Slide the spare plug you acquired on the female terminals, press the white lock-pin and connect the switch.

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50746-switch-relocation-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)

Now reconnect the negative pole of the battery, test that the rearranged/relocated switches perform / illuminate as expected, reattach the stereo and the trim pieces and you are done!

Result
Top and side view:

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50754-profile-view-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)
Day and night wide/zoom view:

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50762-day-night-view-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)
 

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Discussion Starter #2 (Edited)
To "attach" the devices on the magnetic mount, I had to find a way to add something with magnetic properties onto them.
Devices with removable back covers are the easiest ones, as you can place a thin piece of metal between the removable cover and the battery.
Just take care to insulate the metal side which faces the battery to avoid any short circuits and make sure that the added material does not exert additional pressure on the battery.
Batteries can explosively fail under pressure and damage the phone or, worse, provoke injuries.
Alternatively, a metal plate can be placed between your device and a case. The thinner the case, the better because you want to minimize the distance between the magnet and the metal plate as I explained earlier.
If you don't care about looks, you can tape the plate directly on the back casing of your device.
Use as much magnetic material as necessary for a secure hold, but don't overdo it. Metal will adversely affect the signal strength, especially if it covers the antennae of the device.
Below there is an assortment of suitable materials (washers, snipped tin can, razor blade, metal brackets etc) and a depiction of what I eventually ended up with:

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50770-metallic-inserts-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)

Various shots of the end result:

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50778-day-view-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50786-night-zoomed-view-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50794-night-wide-view-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)

(http://www.rav4world.com/forums/members/albums/37389-mick-hatzo/my-mods-8714-picture50802-night-wide-view-2-http-www-rav4world-com-forums-94-4-1-d-i-y-modifications-244865-diy-smartphone-tablet-magnetic-mount-html.png)
 

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Discussion Starter #3
Some pics with the reverse camera engaged, image relayed to a smartphone (5.3" screen, 2560x1440 resolution, guidance lines enabled) and a tablet (7" screen, 1280x720 resolution, guidance lines disabled):

(backup camera screen.PNG)
 
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