Some scrounging around, and I got hold of a flexible strip of plastic that was large enough to bend into a ring - there was the necessary stiffener. After temporarily fixing it with tape, a few drilled holes and some kevlar sewing thread (yes, we have kevlar sewing thread, and it's really nifty!) make sure it will not fall apart in the future.

The ring is attached to the top of the tube with linen yarn and whipstitch. Of course I ran out of yarn three stitches before the round was closed...

Then it was almost done. Almost... but not quite. There's some nice, bright green colour missing, I think!