These 18mm motor retainers work with BT-20 body tubes. Thicker paper tubes can be sanded to achieve a fit.

Full set weight : 0.13oz/3.7g. For use with all 18mm motors, including Estes/Quest 18mm A-C motors, and Aerotech single use and RMS 18mm motors.
No tools required to operate. Motors without thrust ring are friction-fit.

The video below shows just how easy the motor retainers are to use :