Here is a mounting bracket for a Pololu 37D gearmotor. It fits well; my motor sits in the bracket socket snugly and the six M3 mounting screws on the face of the gear assembly were aligned with the holes. The motor can be placed in either of two orientations, with the output shaft high or low. I also uploaded a model of the motor that fits in this bracket: https://www.youmagine.com/designs/pololu-37d-gearmotor