I recently had some cat work done and had a small shooting range cut into the hillside above my house. As we're on well water and our well is down below our house I don't want to be shooting lead into the embankment. I'm thinking about building a box with a trapdoor in the bottom and then fill it with sand to use as a bullet trap. The front would just be plywood, but I'd like to get some kind of self-sealing polymer to cover it so I don't have to constantly replace the plywood. Any ideas?