I recently bought a homebuilt boat and expected to have trouble loading it onto the car, so I made this loader based on a commercial one described on the HBBR website.

In use the boat is placed on the sloping frame and slid up it a step at a time supported by the ali. pegs visible near the bottom of the frame. Then the frame is lifted horizontal and the legs slide along the ground and then vertically to support the boat and frame. The boat is then slid over onto the rack.

Above picture shows hook of frame over roofrack adapter.

Above shows roofrack adapter; the steel strip is from a builders merchant.

Above shows loader in horizontal position.

All parts have to fit across the back seat so the longest parts have a hinge as shown. A spring on the other side (not visible) holds the frame in the "in use" position.
All parts are assembled with M8 stud screws and wing nuts. The springs were not really strong enough and a mechanical device would probably be better. Wood used was teak and oak! But don't worry, it's all offcuts, very little wastage, and was once part of a university lab bench.
I've used it just once, to load a boat (with help) and it was very easy. I think it will be easy even when single handed.