Okay, here's my outside the box method for gluing in motor mounts and motor motor blocks for low and mid power.
I am gluing paper to paper, so I use white glue which gives me a long set time. And since these things tend to stay in place for the moment without any glue at all (unlike a fin, which either you have to hold in position or just a jig or something to keep it in place), once you stick it where you want it, it is unlikely to shift with gravity.
For a motor block, I use a used engine casing of appropriate caliber, place it in as far as I want it to go, and mark the side with pencil (don't want to get ink on the rocket tube), and remove.
I stick my standard glue tip in as far as I can go, usually about half an inch, and do an inside ring of glue, rotating the tube.
I stick the motor block in, and quickly and smoothly advance it with the used casing to the appropriate depth/pencil mark, and promptly remove the casing. With WHITE glue I have not had any problems with casing getting stuck. I have never felt the need to rush, but I am not going to go answer a phone call in the middle of the procedure either..
Does this leave a bit of residual glue on the inside of the tubing below the mount? Sure does, but it has never been a problem for me. Would be a BIG problem if I used polyurethane/gorilla glue.
For motor mounts, I do the same thing, except after I get the first centering ring in past the glue ring, I quickly do a second white glue ring along the inside of the tube below the upper ring, then push the motor mount forward to the appropriate position.
This has been my routine for about 10 years now using WHITE GLUE, I have never (knock wood) had an motor block or motor mount lock on me or fail under use, and I haven't had a problem with the residual trace of white glue in the tube that prevented me from easily inserting an engine. I DID have problems with premature "grabbing" or removal of the casing when I tried this with wood glue.
I also have stopped using store bought motor blocks/engine blocks. I cut a ring of same size body tube as my motor mount about 5mm long. I cut a tiny segment out, so it fits perfectly inside the tube. Tiny piece of masking tape to keep it as a ring. That's my motor block. For low and mid power, have yet to have one fail (again, knock wood.)
YMMV