Today, I visited Hobby Lobby to acquire the A10-3 motors required for my MIRV flight. I also wanted to see what I could dig up to use as a ring fin. Well, I vaguely remembered seeing cardboard boxes that might work. I found that these were made from paper mache and came in various sizes. I opted for a 6" version the actually is 6.5" D and 7" H. This is a tad too small but the next size up, 8", was off more. I have to remeasure but I think I will have to trim the Super Alpha fins by a mere 0.185". I will also have to decide how long said ring fin will be.
Given an air gap of around 0.75" and a 1.67" diameter induction tube, loading the motor may be a problem. So, my next challenge will be how to mount an induction tube to the 24mm retainer cap. I'm thinking I will also need supports between the induction tube and the ring fin.