Used this motor mount on DollarTree foamboard. Simply sandwich the mount on the foamboard -- apply glue or tape to secure it, and you're set. Made this from ABS and was able to screw the motor directly to the mounting plate. Very sturdy and simple to install.
Sized for motor mount brackets with a diagonal hole spacing of 1.3" and 1.66"
Printed out of ABS with 20% infill, no supports.