Maybe a plane to adjust the rails?
You know in reflecting back - as a master woodworker Ed, you know the medium offers great flexibility. If a person wanted to test outline shapes you could create five or six boards in a few hours. With the first board make the wide point one foot back from the nose and the shift this wide point one foot back (or whatever increment) closer to the tail with each iteration.
Another item you could easily test is fin placement. Shifting the fins from the tail FORWARD - say at 1 inch increments. Also, shifting the toe-in, perhaps in 1/8 inch increments. All it requires is a drill and a sticker to cover any holes, or a dab of bondo if you want a more permanent fix.
Not that anyone would stay with the plywood flyer, but for rapid testing of specific design elements, I don't think you can beat it.
Ed S, thanks for conjuring the initial Surf This and Skim That ideas. It was great fun and I think helped me understand substantially more about the hydrodynamics of a board behind the boat.