The mount I made for my NRS and Avon boats is galvanized electrical tubing with an ID that takes an NRS fitting.
It consists of two parallel tubes with a plywood transom bolted into a series of holes (for vertical adjustment) on the short leg. It was bent at an electrical shop and with the NRS fittings, it clamps right to the rear frame member. Total cost about $15, IIRC, mostly to have a guy bend the tube. Still rests on the rubber, but all the thrust goes straight into the frame.
Great idea. There is a significant difference in mounting a motor to a cat than to a self-bailer like mine. Gordo New