bike comp magnets & disc wheels

Glue.

I went to K & J Magnetics and got 25 little rare earth magnets (more than I will probably EVER need). They're about the same diameter as the magnet that goes on the spokes, but thinner than a dime (okay, two of them come to about the thickness of a nickle). I got some of those little foam "mounting squares" that they use for pictures (double backed thin foam) and mounted the first one right onto the disk. Then I put a second one on top of that, then used a little piece of clear packing tape over it (sliced JUST right to form over the magnets). Works perfectly. Total cost was something like $10 for enough to do 15 wheels...