I sincerely regret my decision as a 10-year-old to go as an X-ray machine and not a U.S.P.S. post box. Look how hilarious it is! You can "mail" candy to it which is genius. I love it. There's a rather cursory tutorial available here, but honestly, just get a box and paint and some posterboard and you should be able to put it together. My Internet search turned up a Canada Post version as well. Their box design is even simpler, lacking the rounded top of our blue boxes. I'm sure you could make all kinds of postal boxes into costume inpirations. Send me a picture of you or your victim friend in a postal costume and I'll send you a Halloween treat!