So, I've got a way aroound the number thing. Staples sell vehicle magnets. Just put your #'s on the magnets, and cut them out, then you can take them off. I've got all of my sanctioning body stickers on magnets so I can swap them out if I'm doing EMRA or SCCA or BMWCCA. I also found that you can move your magnets around to hide the blemishes caused by off track "experiences". And, they haven't fallen off even after going into the tire barriers.

If it helps...

R