Here is an idea. Get the bumper sticker, slap it on some Saran Wrap. Than slap that on to your tank when you need it...
Magnetic material should work well on steel tanks as well... Sign shops sell it by the foot, or in a pinch it's available in the form of "duct covers" at the local home improvement store.
--travis (who has raced two different plastic cars, and had to figure out ways to mount numbers when everyone else just uses magnetics).
