Rare earth magnets.
Traditional ceramic magnets can be considered earth magnets.
The best rare earth magnet is made of neodymium-iron-boron
I've done business with this company in the past - great service/company
(link takes you right to rubber-coated magnets in the catalog)
The magnet idea is a good one, I like it.
ETA: an earth magnet and rare earth magnet are not the same thing. Your run of the mill ceramic magnet isn't very worthwhile. Just wanted to clarify what I said right above, it seemed to me like I was saying a ceramic magnet and a rare earth magnet were the same thing, and they are not.
A magnet composed of rare earth materials is much much stronger then the ceramic magnets you can get at your local radio shack. Beyond the neodymium-iron-boron combination, some also have Cerium, Lanthanum or Praseodymium. They are good magnets too. I have had issues with the Hydrogen rich magnets not performing up to snuff, but we might be moving beyond the worth of this kind of information to the thread. Safe to say any rare earth magnets you get will work and be awesome.
Do not cut or drill a rare earth magnet. The composition is plated, and once the plating is damaged the magnet starts to break apart. The rubber coated magnets will last you much longer, but once the rubber coating is damaged be careful in using it.
Care should be taken when using more then one rare earth magnet together. They snap together with enough force to not only damage themselves but to also hurt you, such as pinching your skin.
I still think the magnet idea is a damn good one though. I'm a little put out that I didn't think of it actually, for all the work I do with magnets...