[rwilson452]

you can use a pile of dirt or sand. I would submit you want it at least 6' X6'X3' thick at the top/ if you want to go to the extra trouble you can use plastic bags to make sandbags arrange them in a U with the open part facing toward the fireing line. fill the U with dirt or sand. as others have mentioned beware of large stones in your fill. nothing bigger than 1/2" going smaller is better.