Cool idea but most of the RC bodies are pretty small and all are made of plastic. The largest one I know of is the New Bright and even that would be too small to make a mail box out of. Guess you could glue a NB body to the top of a metal box but not sure it would fit.
Maybe just take a normal box down to your favorite air brush artist and have him paint a mural on the side.
Or grab a good picture of yours and go to a sign shop and have them print it out in vinyl and put that on the box.
Or visit someone who has a plasma cutter and cut a side shot out of metal and rivet that to each side.