Simplest...
Put a large blob of silicone on the bottom of the boat, and press the motor into it.
My first boat held the motor that way for years.
Simple.. Epoxy a block of wood on the bottom of the hull and then the blob of silicone caulking.
Also simple.. epoxy a block of wood with two slots/notches across the bottom face, to the bottom of the boat. Once cured, slip some plastic zip ties through the slots, then zip tie the motor to the block.
Simple metal.
cut a square of metal and drill two holes near the bottom then drill a hole higher up in the center that will clear the bearing of your motor can. Mark two holes for for screws to hold the motor to the metal plate.
Drill out these holes and verify that they match the mounting holes on the motor.
Put the bottom of the plate in a vice, and hammer and bend the plate so that the two bottom holes
can then be used to mount the plate to the bottom of your boat. A block of wood with blind nuts set into the bottom and epoxied to the hull can be used as a mounting point for the motor mount.
I am sure only some of that made sense, but I am pretty sure these all work.
I have used each of them over the years...