You know sand could still be used as a partial solution. I noticed something when I did my rubber crump box. I used several layers of Fedex boxes about 1.5 inches deep to make each layer of replaceable more often (ie fronts ) and to reduce the flow of crump out and away formt he center.
Well I noticed the boxes started to link to at the bullet holes, where card board tubes just about started to appear where the bullet blew materials from one box into the other but in such a way they didnt leak between. So if the front couple of boxes were crump and the rest sand that might work to grealy reduce outflow of sand and help keep the sand from changing the front into Non-Newtonian fluid where you might get a ricochet.