Cardboard rhymes with:
hardboard, hardboard, hardboard, headboard, aboveboard, baseboard, billboard, blackboard, checkerboard, chessboard, chipboard, clapboard, clipboard, containerboard, dartboard, dashboard, fiberboard, fibreboard, fingerboard, floorboard, hardboard, headboard, inboard, keyboard, linerboard, mopboard, onboard, outboard, overboard, paperboard, pegboard, plasterboard, pressboard, scoreboard, seaboard, shipboard, skateboard, smorgasbord, snowboard, springboard, storyboard, surfboard, switchboard, wallboard, washboard