The following mount was for an Estes Laser Lance kit. The lower body tube is a BT-55 and the payload tube is a BT-60, both very strong tubes. The kit engine mount is for 2 3/4" long C11 and D12 engines.
I've seen some very clean cuts on these rings, this one wasn't. That rough edge might catch on the engine hook.
To prevent the ring from de-laminating when sanding, some medium CA was applied to the rough cut with a Q-tip. This hardens up the end cut for a smoother sanded end.
The Great Planes Easy Touch sanding block with 220 grit easily reaches inside the edge.
The upper ring was starting to de-laminate.
Some white glue was slide underneath with a knife blade and the lifted edge pressed flat.
The instructions show both rings even with the ends of the 3" long engine mount tube.
Centering ring ends are typically cut at a slight angle. The outside surface of the rear ring will be exposed. Sand it flat so it will be flush and square when glued even with the end of the main air frame tube.
Apply a coat of CA (see above) and sand the exposed lower end flat. (left)
The dried and sanded CA glued ring edge will end up like hard plastic.