I was thinking about how to make the sonotube as round as possible without having to cut a circle from plywood to exact dimensions to fit it inside so as to bring it to roundness. I thought about metal hose clamps which I heard you can order in large diameters but I found another possible solution without having to order and then wait for delivery of a large hose clamp.
I was at a Michael's craft store looking for small glass jars to hold a particularly strange spider I found when a thought came to me. Knitting rings! I went to that section and I found them in several sizes. I bought an 8" and a 9" ring and ran home. ( I had to settle for an over priced, cheap plastic box for the spider)
The 9" ring (outer) fit almost like a glove around the 8-7/8" sonotub's outside diameter and when tightened made the sonotube as round as I think it's gonna get.
I will see if this solution will work at the open end of the tube as the mirror cell should bring the tube into roundness at the bottom end. Probaly the spider (not the one in the plastic box) will help shape the sonotube but I will see if the knitting ring offers any extra advantage.
If nothing else it's an interesting application for a knitting ring.