Just a thought. I paddled flat water training boats a few years ago that belonged to a local club. They were used by a very diverse group of people. The manufacturer had allowed for this by making a light fiberglass seat shell and then velcroing it to the bottom of the boat. This allowed for fore and aft trim. The seat grip in that type of boat is not a big factor so the fit wasn't butt confroming. What about velcro with a couple of interchangable seats? You would probibly need something wider than 1 IN velcro to keep it in place for rolling but maybe thats not an issue. Steve