What kind of CA did you use to construct your motor adapter? The thick (aka "slow") kind is the stuff you need.
I use nothing but CA to build all my high power rockets. I've also used it to assemble motor adapters. Currently I have several 2.6" diameter rockets with 38mm adapters in 54mm motor tubes.
Here's how I assemble a motor adapter:
I position the centering rings on the motor tube, then dribble a little thin CA on the joint to tack the rings in place. Then I add a fillet of thick CA on inner side of each ring. Next I put the larger tube over the centering rings, and tack it in place with thin CA, followed by a fillet of the thick stuff on the outer sides of the rings.
This has worked fine for me so far.
When assembling the rocket itself, I always mount the fins to the motor mount tube. Not only is this create a tough, durable fin assembly, but it also means that the centering rings don't have to take the full load of thrust.