well....i've always had those cheap chrome license plate covers you can get at pep boys on my back plate, but they never last long and i'd end up replacing them every couple of months. so i decided to try a nicer and more expensive cover
it completely seals the plate inside. the chrome is also supposed to be very thick and hard. last, the cover is real glass coated with some very hard plastic.
here's a few pics (it rained today, so my car is dirty :( )