I designed this motor mount to mount a 12 V DC motor to a piece of plywood. The motor can be found cheaply on ebay here
The dimensions of the motor are as follows
Shaft diameter: 4mm
Product dimensions: 25mm
I made the hole in the middle 5.5mm to suit the fixed spacer on the
front of the motor and it fits snuggly and would hold in place without screws for light applications.
I printed it face down in pla with 10% infill with supports to support
the base lugs with a raft and it took 39 minutes.