No mention of trying duct seal or rubber mulch in your post? I guess you must have missed those over the last 25 years while you were chewing up 2x4s and plywood. I've been using a duct seal trap for about 25 years, and other than digging out fused lead lumps occasionally, I've only had to partially refill it once. I have a rubber mulch trap that I shoot a .357 airgun into, and I just tape over the holes in the front occasionally. Personally, I'd gather not have kitty litter or sand dumping out of holes in the front of the target.
The kitty litter in a somewhat self healing container is a nice idea. But kitty litter is available in square sided plastic containers itself. When one side gets too shot up just tape over the leaking holes and rotate ninety degrees. No need to save a different container to use and then put kitty litter in.