I know it's not a bullet trap, but it could be used as one. I shot an old trash dumpster that had been hit by, from the looks of things, a semi. I didn't shoot the sides just the bottom. The 230gr FMJ just fell to the ground. It flattened the bullets out like they had been put in a press. I was thinking that building a wood box and hanging a piece of three eights to half inch steel would work for most cast loads.
http://www.wvpoa.com/images/dumpster2.jpg
By the way it also stopped a 318gr 44 magnum bullet too. Both rounds were fired from six feet.