go to hardware store, and buy those white magnetic sheets you put on the vents for central air. they come in a pack of 3, for like 9 bucks or something. just cut out your #'s and your Class and you're good to go, just stick them on your doors. make couple diff # configurations in case your # is taken. remember you need these for both sides of vehicle.
it's a PITA to keep making #'s using tape, or isn't clean using polish on windows.
Or, you can print it out on paper nice and large, and just tape around the 4 corners to the door.
