Alright, so I found another solution to temporairly customize my car. You probably have an equivalent product, or even the same product in the States.
http://www.pebeo.com Arti'stick Window Color. What's cool about this one is that you can apply it on any plastic or glass surface, and after 24 hours, the dried paint can be peeled off and sticked anywhere else. On June 24th, our National Day, I plan on drawing a Québec flag in white and blue, maybe with black borders. I'll prepare it the day before to stick it later.
This will also tell my if it's sticky enough to stay on the outside of the car for a whole day or more. Let's hope so