I went to a local artist's supply store (Dick Blick), and bought 6" vinyl stick-on numbers. A pack of 2 each of all digits was about $12.00. That way I can come up with numbers no matter what I am assigned. I use them on the rear glass, next to the door handles. I have successfully but carefully peeled them back off after use, and stuck them back on the wax sheet and have reused them. I tend to get 2 or 3 uses before the stickyness wears out. These numbers are on a single sheet, so you peel them off and then apply them.
The numbers shown on my picture to the left here are the numbers they give you at Road Atlanta at a Panoz track day. They are similar vinyl stick ons that can be reused, but they come between 2 sheets, so you peel off one, rub it on the glass, and then peel off the 2nd sheet. A bit easier than the single sheet jobs.
Jeff