The tailcone is drilled through to fit a 10mm tube, which is a BT-4. The motor mount tube is a BT-2, 1” length. BMS does sell BT-2 and BT-4 tubes but only in 34 inch lengths. They also stock the BT-2 to BT-4 centering rings (eRockets also has centering rings). I chose to mount my BT-2 motor tube into a piece of BT-4 tubing, then slide the whole assembly into the balsa tailcone. You could alternately skip the BT-4, and use paper stips and glue stick to wrap your own centering rings that fit the balsa tailcone. Your choice. For the motor stop block, I used a small laser cut block I got from Ollie-OOP Rocketry. You can also use BT-1+ tubing for this purpose, available from eRockets. The kevlar is 100# waxed Kevlar I got from the kite store on a roll.
I slid rings on motor tube and glued them in place, glued the stop block into the motor tube, tied the Kevlar between the centering rings and gave the knot a drop of glue to set the knot. Once motor tube and rings were ready, I slide them inside a 1” long BT-4 tube and glued them in place. This is the final motor assembly that will go inside the tailcone.