It may depend on how much attraction you'd like to have. The black plastic sheets can easily be cut and could be mounted with the glue backing that is usually on them, or double sided foam tape for added strength. You'd need a decent sized square to make it hold.
Rare Earth magnets are they ones you normally find on the back of clips or plastic covers that hold notes on white boards, refrigerators, etc. A step above the sheets, but for good attraction you'll need a fairly large one (think refrigerator door magnet size).
Neodymium are stronger than the other two and would be too strong if they are the same size as a normal refrigerator door magnet. The advantage could be that you can get a smaller one (maybe 1/4-1/2" across) that would have a reasonable amount of attraction. Depending on the vertical gap available, you might be able to surface mount one with a small steel washer on the other side.