For scale, the rings are about 9" in diameter. The channel section is 70 cm or about 27.5" long. This length suits both the straight and the folded OTAs.
Hopefully I shall be able to use an electric jigsaw to speed up the process. Provided the cut and the blade are kept well oiled a jigsaw, fitted with a metal cutting blade, works well on aluminium.
I am so sick of waiting for an intelligible response from the flange dealer that I am going to take the compression bush route. RS.dk sells a 50mm locking bush for about £60 equiv. This uses opposed cones and screws to tighten itself firmly onto the shaft.
I need a strong connection between the saddle and the end of the Dec shaft. Even if I waited for the flanges I'd probably need to have them welded to the shafts. Even then I'd need to cut two 'ears' off the heavy stainless steel flange to fit inside the channel. Welding takes it beyond the unskilled, self-assembly brief I have set myself. The difference in price for the bushes compared with the larger flanges is peanuts.
The tension bolts can be used for saddle retention to the 92mm diameter head. By a happy coincidence, 92mm matches the internal breadth of the channel section saddle rather nicely.
By using all the tension bolts with load spreading washers I should obtain a seriously stiff connection. It may be that the ring flange is supposed to sandwich the intended load. Which is all to the better. The tapered channel section can be tightly packed with birch plywood to further stiffen it. Channel section is weaker in torsion than box sections or pipes. More to follow when the bushes arrive.