I did go and get a piece of 1/64″ plywood and I added it as an outside layer on the ring fin. More on that later.
The ring fin is about 7 5/8″, maybe 7 11/16″, inside diameter. Let’s call it the latter, 7.688″. BT-80 outside diameter is 2.600″, so the fin height is to be half the difference, (7.688-2.600)/2 = 2.544″. (I come up with 2.222″ scaling the scanned Sprite fins up to BT-80 size, but my ring fin is a little larger than scale, so this makes sense.) BT-50 outside diameter is 0.976″ implying 0.812″ fin tab height, and I’ll make the tabs 3.5″ long. So here’s what I come up with:
I don’t actually have a measurement of the ring’s thickness, it’s probably about 0.06 or so; anyway, the slot depth may need adjusting.
The fins are going to be made, Mega Mosquito style, from laminated balsa: two layers of 1/16″ sandwiching a layer of 3/32″. (And probably paper sandwiching that.)
I think I’ll cut fin mockups out of cardboard and test their fit against the ring before cutting any balsa. Sound like a good idea to you? Me too.