The last one I made was from a 20"x 20" box from a flat PC monitor and it was about 6" thick. I filled it, packed it solid more like, with more sheets of cardboard. We had just moved, so had lots of boxes around. Anyway, I simply cut each piece to fit inside and layered them in until it was full, closed the box, taped it shut and taped paper targets to the face. This worked great. We fired my crossbow and my buddy's compund at it for a couple hours from 15 to 25 yards with no problems.