I like the idea, but the tinting on my windows is so dark that it wouldn't be seen and if I stuck it on the outside, I doubt it would last long.
Some people are making removable bumper stickers and signs, either with magnets, or using some kind of frame. You have to remember to take it off at your destination, to avoide the Dreaded Liberal Car Key.
You could just put it under the windshield wiper of a car with a SKerry bumper sticker.