Only problem is if it rains the cloth would soak up and get wet and the glue would come off.

Magnets will not scratch your car UNLESS you move the magnets while it's on your car. You just gotta remove the numbers and letters instead of sliding it around while it's on your car.

There's an easier way to make numbers. Go to an arts & craft store and purchase sheets of magets. Cut them into little rectangles (guessing 2"x4") and you have an infinite amount of numbers and letters you can make.

Sort of looks like LED signs.