Hi Matrix,
We made a roof rack years ago for a Rainbow for our trusty old Kingswood wagon using steel poles, like tent poles. The arms holding the dinghy slid out to the side of the car and then pivoted to sit on the ground, so the dinghy sat at 45 degrees so to speak. Lift off the dinghy and off you go. It worked like a charm for a few dinghys over the following years