I build all my 6 by 6 inch box beam strongbacks from 5/8 ply with spacers inside for strength. All of them are on two by four stands with wheels as I also need to move things around my shop and outside for sanding on good days. They remain quite straight. This set up is for external beams in which you can adjust more for beams that come out of true over time. Info is on my web site on the How To page. http://www.orcaboats.ca