You can see that I welded 2 sections together, then cut out the holes for the towers and drilled the holes for the strut mount bolts. Please forgive the shaky hand on the cuts, I will say that plasma cutters are my new favorite tool, but I have a lot to learn about making good-looking cuts with them.
Now, I had some tube stock that I cut down to make a smaller diameter...you'll see why in a moment. First I cut:
Then I pinched it down to the smaller diameter, welded it shut, ground it smooth, and got it to fit nicely inside the diameter of the regular tube stock.