The mount for the 28mm Motor was designed to clamp the motor and then be mounted via two screws tapped into the bottom edge. 

The designed shape would be easy to watercut but I made the prototype on my milling machine.

The shape is slightly different but it works the same way.

Another view.  A M4 x 20mm screw is used to provide the clamping force.

I milled two slots in the bottom carbon fibre panel so that the mount can slide to adjust belt tension.  The mount is secured in place using two M4 x 10mm flange head screws (The large built in washer spreads the load better)