2 of BT80 - 29mm centering ring holds 29mm motor tube inside BT-80 airframe. 1/8" plywood.
Fits into any 2.6" BT-80 tube. Works best with LOC paper, fiberglass, Semroc 29mm motor tube. (1.22 - 1.225" motor tube OD)

One ring has a notch to accommodate G size hook.

Please see 2.6" rings for 1/4" version of the same ring.