If I do that, I'll get my windows broken on my car.
Get some flexible magnetic rubber sheeting: 
Some people worry about damaging their paint. I worried about having pro-gun bumper stickers on a car when I had parked it somewhere with a gun in it.
The stuff makes it easy enough to take the bumper stickers off when you park someplace where you might be at risk of vandalism.
I've thought of that.
Sadly, I drive a Saturn and it's made of plastic ( mostly)
thanks anyway!
This idea was great when bumpers were METAL.
Now all this crap of plastic and fiberglass - it has to be a very strong magnet to "feel" the steel way underneath past a layer of air, too.
The really sad thing is all this stuff is painted along w/the rest of the body. You are really screwed when you take off a bad bumper sticker. My car is all scuffed up thanks to it. Metal/chrome bumpers, ironically, made removal easier and if not, it didn't hurt much and you just cleaned it up.
Modern designs just suck. (Old car nut here, too.)