i built one several years ago out of 1/4" plate, i angled the back down and put a front plate on it, it held up very well to hard cast lead bullets out of a .38/357 i also put sand in the bottom of it, thing worked great but was very very heavy[:@]
here is a link to some commercial traps and the rubber mentioned earlier, i have not shot at the rubber but have heard good things, best of luck, oh yea i always shot in basement[:-]
http://www.letargets.com/html/traps.html