The best backstop material is dirt free of rocks. My club sets up a series of bangplate targets along a U-shaped ravine around our camping area. The shooters stand above, or level with the metal targets, which are placed so that the side of the ravine provides a tall backstop, and because the ravine is fairly narrow, both sides of the ravine prevent any ricochettes from leaving the ravine. Shot are taken either at a downward angle, or at level. NEVER shoot upward, unless there is a huge hill behind the target to stop misses.
We also mount the plates so that they tip forward, towards the shooter about 15-20 degrees, suspending the plates from about 1/5 below the top edge. This allows the full weight of the target to act as a stop if a shot goes high and hits above the pivot point, so that the balls still go down into the ground in front of the plate.
A controlled ricochette can be the best way to deal with ALL ricochettes. They do happen, even in the best of circumstance.