This would actually work with all the motors in the same line as the GM18, but I don’t know if the higher torque motors would cause problems with how the motors are mounted. In any case here’s the trick:
Turns out the motor can on the GM18 is just slightly smaller than the inside diameter of schedule 40 CPVC pipe. (Sorry, I don’t have it in front of me so I can’t say which size pipe, but basically take your motor to the hardware store and see for yourself.) It’s a close enough fit that you have to have a pretty clean, deburred edge on the pipe.
To make a tube-style motor mount for two of these I cut off about 2" of pipe, faced each end square in the lathe and brought the length down to 1.7". The motors slid in each end until the back plates of the gearboxes came up against the faced ends of the pipe. I then drilled and tapped two holes in the side of the pipe to take a #4-40 set screw to bear against the flat side of the motor can to hold it in place. Since I didn’t want to risk damaging the motors I stuck in a piece of brass shim stock between the motor can and the set screw. Works like a charm.
To gain access to the motor wires I drilled out a large hole near the midpoint of the tube and snaked the wires out through there.
Attaching the motor mount to the robot frame is up to the builder. One easy way would be to drill and tap some more holes in the mount so it can be screwed into the frame. Since the frame of the robot I’m doing this for is foamcore board, I’m going to simply glue the tube to the robot frame. The motors can still be removed for replacement or servicing, and the motor mount will become a permanent part of the frame.
Not sure how useful all of this is, but I’m tossing it out there for what it’s worth.