Hi Larry, I am currently building the Expedition Single. I built a box beam strongback out of 3/4" scrap plywood. I constructed it in 3 sub-assemblies that screw together. All joints interlock like a puzzle. It is plenty strong and straight as an arrow, plus breaks down into 3 pieces for storage. I built it 2" x 4" and cut the forms slightly oversize. Ended up having to re-work the taper for a perfect fit. I can't imagine a better way, it's much stronger and straighter than any solid wood 2" x 4", much cheaper and easier to modify than aluminum. Only drawback was the 4-5 labor hours required to construct it.