My riding sail didn't work terribly well, partly due to its shape and partly due to being too far forward. It was run up in a hurry by a friend without much regard to shape, and I'm sure a better shape might help. His works brilliantly, but he has a ketch and it's way back on the mizzen.
When I finish my current agenda of projects I'll experiment with one attached between the back stay and the solar mounting pole, which is supported at the rear of the pushpit so it will be a bit further aft. Now that I have an industrial sewing m/c I'll be able to run up some trial shapes in any old fabric and see which works best and then hopefully make a fair-dinkum one.
Cheers, Graeme