Did the same thing, but didn't compress the cardnoard really tight.friend made his with 2x4s, threaded rod & cardboard strips cut about 8" wide. take 2 pc 2x4 about 3' long, drill 3/4" holes about 4" in from ends. cut cardboard strips to fit between holes & stack them up about 3' deep. put threaded rods (with nuts one end) through holes in 2x4s, & put nuts/washers on other ends. compress cardboard till tight! when you shoot out center, beg, buy, borrow or steal more cardboard as needed.