Thanks much! More work than I thought, but I had to figure out some of the tool workarounds as I went.

I cut the balsa ring down the middle by using double-sided scotch tape and some balsa sheet to secure my razor saw to a block of balsa, with the saw blade exposed to the side. Got a dead straight cut, and after hardening with thin CA and sanding a couple of times, the faces are perfectly flat and slide silky smooth past each other.

Ah, okay the last pieces of the puzzle. All I need now is some 1/64 ply. I have the bike tube.

Thanks again. Would not have been possible without your explanations.[sm=thumbup.gif][sm=thumbup.gif]