I wonder if a heavy steel plate at about a 30 degree angle would work. As far as sand depth, maybe about a foot or so. I've thought about making a trap too.
I made one for a man that cast & loaded for law enforcement training , he started with 55gal. drums full of sand but they did not last long .I made a 4'h x 8'l x 2' deep box with out a top made brackets that he screwed used roofing tin to . he said the sand did not turn the bullets to powder like a steel trap did . mite be something to think about
I have a 500 gal. barrel laying on side filled with sand.....probably 6-7 feet long. Do not save lead. Barrel is covered with pile of dirt....kooks like a big mount of dirt. Catches anything ever been fired into it. Place a target in front of opening and range is 300+ yards. Been using it for many years.