Any body tube size will work. I'd say scale the picture for a BT-20. Any nose cone will work. Use the scale for length and size of the ring. Not sure if Bruce split the ring, I just put the inside ring on the outside diameter of the BT-20. I used two launch lugs, one on each side of the BT-20 / Ring joint, for strength. You could go with on lig and replace the other with a 1/8" balsa strip (aka a "cheater") Quaker cardboard round oatmeal canisters (not the cheap generic cannisters), pringles cans, and mixed nuts cans (which are usually cardboard with a metal lid and base) can be substituted for the ring.
Streamer recovery is great, the ring has so much drag, it comes down slow anyway once you blow the nose.
@hcmbanjo @Daddyisabar , here's an interesting question: I know real research sounding rockets occasionally used canted fins or even a helical launch tube to impart spin to rockets to minimize weather cocking. Would corkscrew rockets also be relatively immune?