I obtained rubber motor mounts from MFA/Como. These have 3mm studs at each end and as I had some 3mm threaded hexagonal stand-offs to hand I decided to use these for mounting.

As it turned out the height of these plus the rubber mounts was about the right height to just stand on the bottom of the hull. I actually contemplated fixing them with a screw through the hull, but not for long. Instead I cut some 15mm copper tube to make little cofferdams which I superglued round each mount location. With the mounts assembled to the motors I then put a bit of epoxy in the bottom of the cofferdams and stood the motors in place, making sure alignment was correct of course. When this was set I removed the motors and rubber mounts and poured some more epoxy around the stand-offs and also put a fillet around the outside of the copper tubing to ensure everything was well secured as seen here.