I have thought about this as well. If I ever get a big enough piece of land, I would like to set up a place for me and friends to shoot. My main concern was the possibility of a gun newbie shooting OVER the mound. I've seen many rifle ranges with wood posts and planks every x amount of yards design to catch any bullets going too high.
As for the dirt, a 2% slope on 200 yards might provide enough to make the mound.