Dirt doesn't cause ricochets like gravel/rock can do. Use dirt and plant it with grass to hold it in place.
If you think you might still get ricochets, directly behind the target area you can erect a vertical backstop of thick exterior plywood and backfill behind it. When it's shot out, nail another sheet over it. By the time that one is shot out, the backfill should be set enough to pretty much stay in place. Leave the old plywood there indefinitely.
Besides, if you make a 300 yd range, the holes in the backstop still won't be over 6" wide, right?
