Use your car's back window. These are neat decals that you can see through. There are probably lots of services that do this sort of work, but this is what we found . . .
http://www.dezignwithaz.com/custom-window-through-decals-p-2461.html
Here is an image that would look great!
It's crisp, it's clear, and the message is simple. Or use your own image!
