Magnagrafix.com made a nice set of magnetic numbers for my 911 when i was doing DE's , they held up very well although i found it best to put a few strips of thin two way tape on the underside as i did lose one at one point and it was pretty mangled when one of the flaggers brought it to me at lunchtime !

Cheers
Phil

 

I used Magnagrafik and they are great, lots of styles. I have not had an issue with them coming off even at 130 plus...

magnagrafik.com

 

Silly question, but assuming you don't want permanent numbers, why not some white scuff cote shoe polish? Wax on, wax off.

$1.49 worth will last you about 5 years, and no stress...

 

Correction:

http://www.firestormgraphix.com/viny...c-numbers.html