Here is the whole run-down.
i used a poster board that had really faint lines on one side (super nice for measuring) and the back was just blank. i have a piece that is placed on the bottom and curves up against the back wall of the box (standard issue cardboard box 16"x16"x16") and then i have a piece of the poster board on the top panel of the box which the flashes are firing against. everything is taped together with some crazy strong tape (kind of like gorilla glue tape but a poop green color). the flashes are held against the side walls with shoe laces. two holes in the side walls for the laces to go through and wrap around the flashes.