This is nice but probably much more expensive and extensive than I could swing.....
You don't have to be this elaborate with your setup. Pick up about 8-10 used tires of the same size along with a bag of 1/4" nuts and bolts. Some chain tire shops will give you the tires if you tell them what you are doing with them, and tell them that they can shove a knife through the side wall, if they would like.
Build a carriage for the tires out of some used lumber and bolt the tires together as you set them in place. The inner walls of the tires create a baffle system, like a jolly green giant sized suppressor. You will be surprised how much it quiets your shots.