Your shady tree mechanic is lazy & worthless if he can't cut some screen and zip tie it to your grill... If you have any mechanical skill, surely you can cut some screen and zip tie it in...

 

I think the best way is to buy the top and bottom guards from Voltstuff. I just did them. Easy and fun, and cool to support a nifty small domestic business.

To answer your question, the bottom guard is just ziptied to the outside. The top guard is ziptied inside, you remove the hood latch, the plastic gap cover thingie, and the horns, and it just slides in place with a little finagling.

 

You can get a cheap, lifetime supply of plastic gutter guard from Home Depot, Lowes or Ace Hardware. It's black plastic and comes in a roll. It is much cheaper than the professionally made guards but works just as well. I don't want a heavier metal screen hung off the nose of my RAV. The plastic is easier to cut (use scissors) and install than the metal variety of hardware store gutter guard. If it ever gets broken or damaged you just put on a new piece.You cut off a piece from the roll long enough to go over the A/C lower grill and then FOLD it in two lengthwise to give a double thickness.Then you just poke plastic zip ties through the screen and grill to hold it in place. It's a fiddly job and a bit of a pain but not really that hard. I had a large rock hit mine and dented and snapped parts of the screen on the outer fold and that rock was trapped by the inner fold. I quickly replaced it with a new piece. I never bothered to install any screen for the upper engine coolant radiator as I have never in many years of driving (including off road) had a radiator holed by a rock. Maybe I have just been lucky, but on the 5th gen RAV I have not read any posts about owners coolant radiators ever being rock damaged--just the A/C lower condenser . Here's a bunch of photos of different RAV4 A/C grill guards... some plastic; some metal; some professionally made.

 

I have used the Voltstuff twice now and find it an excellent product. It is SS so should last forever. Plastic can be damaged by UV light.

Recently, I had a real rain gutter issue and found this SS mesh that could be used like the others on the RAV4: Amazon.com: Valchoose Upgraded 304 Crimped Wire 0.65mm, 16X144 in (407x3660mm) Rust Proof Hardware Cloth Sturdy, Wire Mesh Roll Easy to Cut Shape, Chicken Roll for Chicken Coop, DIY Cage : Industrial & Scientific