Here's my motor mount, and how it's installed in the hull. I made a 1/8" thick plywood platform with blind mounting nuts and painted it. Having the blind nuts lets me use hex head screws, so the motor can be removed and reinstalled with no loss of strength. It's glued into the hull with 3M 4200 white sealant. The motor mount came with the Robbe geared motor. It's a thick piece of aluminum sheet, bent at 90 degrees and machined for the fins on the motor. I use a thin rubber sheet between motor mount and plywood, which cuts down vibration and noise. Having this type of mount will allow me to change motors, as long as the new motor has a shorter centerline height, and then it can be shimmed to align with the prop shaft.
I have made a similar mount for my Springer's motor, and no problems after 3 years hard running.