I want to make some 4" x 4" x 4' support "boxes" to raise a Table Top glue up above my work bench so that i can get clamps and cauls under the glue up. What materials should i use for these 'boxes'? I have a piece of birch plywood kicking around that is 2' x 4'. I believer I've had this for at least 6 months and I've been told that plywood is a stable material but this piece already has a curve to it. Any suggestions? Thank you!