I have always used motor cases with the AeroTech style aft closure but I recently picked up a Kosdon 29-150 and don't really trust friction fitting so I thought maybe someone here might have some suggestions.
I assume you're using motor-ejection systems, rather than electronic...?
You *could* make the motor-mount tube longer than the longest motor you plan to use, and glue a stop to the forward end. Then, all you need to do is add removable extensions to align the aft end of the motor flush with the aft centering-ring, and use a plate to hold everything together.
Don't Kosdon cases have grooves for external thrust-rings...?