The solution we developed was to make a mounting plate that fit against the face of the motor, with a 0.695" hole for the drive pinion and pinion bearing bump to rest inside. Two 6-32 screws are put in the mounting plate to act as pins that stick into the two round holes on the face of the motor, to prevent it from rotating. Two 1/4" x 1/2" slots are opened up in the mounting plate to allow free airflow through to the ventilation slots in the motor face (more on this below). Finally, two "J-hooks" are made from 1/4" drill rod. The J-hooks screw into the mounting plate, and lock into the little square notches at the back of the body of the motor. The J-hooks keep the motor flush against the mounting plate, and prevent it from translating backwards. You can see how this works in the lower right corner of
this image.
The thing that we liked about this approach is that it requires no modifications to the motors, and no special efforts to prep the motors for mounting. Swapping a motors takes about one minute - just long enough to loosen the two J-hooks, pull the motor backwards off the pins, put the new one in place, and tighten up everything.
I would offer one comment regarding the Lexan "press-fit" motor mounts described above. As shown in the drawings the motor mounts cover and occlude the arced slots in the face of the motor (they may actually be built differently, but the drawings are the only references shown here). The basic design behind the Lexan motor mounts is pretty slick. But these slots are ventilation intake slots, and they must remain unobstructed to allow proper airflow through the motor. If they are covered, the motor will build up heat, and you will run the risk of a thermal overload (resulting in the power leads becomming unsoldered, bearings seizing up, rotor colliding with the case, and other bad things). It is strongly recommended that you add ventilation slots to the motor mount that will allow airflow through the slots in the face of the motor.
-dave