First the good part. I cut some Lexan fins. The little plastic tabs were just too small.
Now the bad part. I sectioned a lamp cover and found that it contracted tightly to about 18mm. I thought a bunch of spent 29mm motors would hold it in place while I taped it with clear packing tape. One layer of tape held the motor loosely and it meshed nicely with the outer tube. So far so good. However, the packing tape wasn't enough to keep the tube open. Once the end motor was removed, it began to tighten and the removal of the other motors very difficult. I tried tape in both directions and neither worked. After 2 attempts, I'm punting.
If I had another set of perfectly sized spacers to place every 6 inches or so, that might work. But that is a level of precision is not readily attainable. I'm not going to spend much on this so buying a commercial clear tube (say clear PVC) isn't in the budget (ven if a perfect diameter could be found).
If the sectioned tube doesn't fit snugly, it doesn't seem to add much strength to the body, which was the main point. The final problem was forcing motors out of the tight tube did a good job of scratching up the inside, so it no longer looks crystal clear.
UPDATE: I may have a solution. Since the tube curls to a small size, maybe I can try a T12 FTC tube. When it shrinks, it might serve as a liner to the T8?