I get free 15"x3"x3" blocks from work. I just stack them up behind my target. For the actual trap part I am now using an electrical outlet steel box, the type that handles 4 plugs, filled with duct seal. I already had the duct seal, and the electrical boxes cost me $1.98 at Home De' Pot. I made two, one for the garage, and one for the backyard. I think the boxes are 4"x4" or so, perfect for paper targets, and if I miss I have that stack of 3x3x15's behind it. No neighbors complaining about their dead dog "Barkbark", so I guess I done good:)