Based on photographs on Thingiverse, it seems that the motor mount doesn't account for the flanges on the face of the motor. I would suggest using one of the following features to provide a better load path for the compression load resulting from the mounting screws:
- A boss around each mounting screw hole between the gearbox face and the motor face
- A boss on the gearbox face that fits between the flanges on the motor face and surrounds the mounting screws
- Grooves in the gearbox plate for the flanges to sit in. This option could be problematic since it changes the axial placement of the motor.