I am still struggling with the tube issue. It does seem as though I do need tubes both to position all the parts, but also to help provide some positioning for the square tubes. The difficulty is getting sufficiently round tubes.
I experimented with rolling .065" 5052 aluminum sheet--and developed an increased appreciation for the difficulties in rolling .090" aluminum sheet into a reasonably round tube. It strikes me that it might be possible to start with a thinner sheet--say, .025" aluminum sheet--and roll more layers, something like doing a spiralbound cardboard tube. Four turns would produce a .100" wall tube, which I could then bolt the layers together, at least long enough to get it to a welder to weld the ends and the edge. I'm told that model airplane builders sometimes use epoxy and aluminum to get a result somewhat similar to making fiberglass tubes--and perhaps epoxy between each roll would give the same result. The big problem is that I can't seem to find a local supplier of .025" aluminum sheet that can provide longer than a 48" strip. Four turns on a 20.4" diameter means slightly more than 256 inches.
Trying to get someone local to roll an aluminum tube hasn't been so successful. They either want a pretty frightful amount, or they aren't able to weld the resulting rolled sheet into a tube.
Perhaps I gave up on making a fiberglass tube too quickly. I did so because I thought that I had a supplier of aluminum tubing the right size. Instead of struggling to get fiberglass cloth to wrap around the end of Sonotube, I should plan on using the Sonotube as a mandrel around which to form a pure fiberglass tube.
The technique for this involves applying a layer of polypropylene as a release layer, then painting epoxy on the polypropylene, then applying the fiberglass cloth, then another layer of epoxy, repeating until you have something that is stiff enough for you to be happy with the result. Since the 20" ID Sonotube is about 1/4" wall, I would end up with a 20.5" ID fiberglass tube. The web pages showing how to do this make it sound relatively easy to do--maybe I just need to make another try. Certainly, the materials cost isn't bad, and I still have a good bit of the resin from the last attempt.