I use aluminum for a strong back but it is not cheap. Do a search on 80/20 Inc 15 Series 1.5" x 3" Heavy Wall T-Slot Aluminum Extrusion and you will find a lot of sources. I use left over plastic 4X4 covers from my deck reconstruction for spacers (they were about 18" longer than they needed to be) and I use a router table to make the form holes. It makes a nice tight, stable, heavy, and true strong back without a lot of fussing trying to align the forms. These pictures will give you an idea of what it looks like.