I considered steel tubing. I am in the electrical and mechanical machine design business and have access to all types of metal products. At the time we had 2x4 steel tubing, 24 ft length, in stock. It was not straight enough to suit me, thus the extruded aluminium, which is very straight and strong, plus has built in connecting slots along its length.

I did build a wooden spine first, out of plywood. I just wasn't happy with the floppyness and curvature of the thing. I know it will work, as evidenced by the builders who do this all the time. I need to be able to move my project out of my way occasionally, and preffered to have a very straight and rigid spine from the beginning. On casters (not shown in pics), I can roll it in and out of my garage without any worry.