Second Try With Fiberglass Worked Better
I mentioned the lessons that I learned from the first attempt at making a big tube. While I did a better job of laying up the fiberglass cloth, there were still some wrinkles in the plastic, and the resulting tube wasn't as round as I had hoped. (But it does fit over the mirror cell!)
The bad news is that even with five complete turns, the tube just isn't stiff enough. It's too big to bake in the oven, so I used a blow dryer--which made an obvious difference--then I put in the spare bathroom with a heater set to bring the room up to 85 degrees for several hours. Now it is back out in the garage, with an electric heater warming the room. Perhaps I need more turns of fiberglass? Even though I haven't trimmed it to the correct dimensions, it only weighs a bit more than two pounds. Perhaps five more turns? It would still only weigh four pounds.
The good news is that the form came out very easily, without damage.