Enerjet by Aerotech, Astrobee D Build, Part 25, Rear Centering Ring

With the TTW tabs locked into the fin lock rings, the body tube is squeezed in on four sides.

The rear centering ring is slid over the engine mount tube and down against the back of the TTW tabs.

To make the slide in easier, 

I lightly sanded the edge of the ring on one side to slip over the engine mount tube.

It took some coaxing - 

But I got the ring all the way over, in and against the TTW tabs. 

There is still a slight distortion of the round main airframe tube.

A wood glue fillet was applied drop by drop using a Q-tip applicator. Apply a drop of glue on a Q-tip, place the drop and smooth it out using the same Q-tip.