Enerjet by Aerotech Strong Arm Build, Part 24, Engine Retainer

The inside edge of the motor retainer should be roughed up for better epoxy adhesion.

I used the back side of my knife to scrape off the paint. Using the back of the blade will rough the surface and not dull the sharp side of the knife.

The fit of the ring is tight.

To make an easier slide in and seat, the edges of the motor tube were slightly rounded with some 320 grit sandpaper.

Here's the ring slid in until the motor tube butts up against the raised ring inside. 

After the epoxy dries, the cap can be screwed on. 

The trailing edges of the fins are longer than the retainer cap so the rocket can stand on it's fins.