There was a rocket club meeting last night, and we brought Sprite-like objects. 
There was one nice mostly-assembled 3D Rocketry Circulus 3. There was one nice scratch BT-60 upscale. There were two Semroc clones, mine and a nice one. The other builder hadn’t primed and sanded before putting the ring and fin tips on, and the fins reached all the way to the ring. I still don’t know why mine didn’t.
I brought an 8″ (Nominal) concrete form tube, 48″ long, just to scare people.
For my BT-80 Mega Sprite, I needed a ring fin about 7.5″ diameter. I decided to roll my own by gluing up three layers of poster board. (Fairly thin poster board, 10 point I think.) I needed a 7.5″ cylinder for a mandrel, and I thought I found one in the basement, a part of an ice cream maker. But when I went to wrap my card stock around it I discovered it was slightly tapered.
So I headed to Home Depot and bought a form tube.
Funny thing about 8″ (Nominal) form tube. It comes in four diameters.
I’m just speculating, but I’m pretty sure the reason is they can ship four times as much tubing in the same space this way, by nesting the four diameters. (And those inside various diameters of 10″ Nominal, inside various diameters of 12″ nominal, I suppose.) Of course most people digging a hole and sticking a tube into it don’t really care if it’s 7.5″ or 8.5″. I wanted as close to 7.5″ as I could get, so I chose one of the smallest diameter. I think it’s 7.5″ inside diameter, so the ring I built up on the outside is larger than scale diameter, but close enough.
The result’s pretty good. 
I staggered the join on the three layers by 120⁰ so it stays round. It’s not very stiff, and I’m not sure how much that matters. Of course it’ll be much less floppy when it’s glued to the fins. Club members had a couple of suggestions for further stiffening if I want to do that: Fiberglass and epoxy or, something I never would have thought of, 1/64″ plywood. I might decide to do the latter.
Of course there’s also the form tube itself. My two concerns were with whether it could be cut cleanly enough, and if it would weigh too much. But I weighed the whole tube and calculated, and I think it’d come in at around 45 grams, which is not much more than the weight of the balsa nose cone. Turns out the ring on the Semroc clone is a little heavier than its nose cone, so that might be fine. As for cutting it the member who made a 10x Fat Boy upscale out of 24″ form tube cut theirs with a hand saw with pretty good results. Of course one could saturate the cut end with epoxy and then sand it flat.
Seems easier to just use the poster board ring, maybe stiffened with thin plywood, so I’ll probably go that way. And do what with the form tube? Well, 7.5″ nose cones are hard to come by, so I think it’ll gather dust for some time.
One other thing I’ve done is to cut the BT-80 body tube to 9 3/8″, the correct scale length give or take a few hundredths. 
And I’ve enlarged the fin diagram to BT-80 size (PDF), though I’m going to have to adjust the fin heights to avoid the same problem I had with the Semroc clone.
Rich's Rockets 