I can't imagine a stadium seat working out very well.
Fortunately, my Wave came with the backrests, so I didn't have to decide whether I wanted to spend all that money or not to buy them. They are nice to have when sailing in lighter winds.
I don't see why a guy couldn't make his own backrests using the same concept of putting some kind of tubing in the crossbars. I do see suppliers for aluminum pipe and fittings on the internet:http://www.diamond-aluminum.com/fittingpages/105.htmhttp://www.gemplers.com/product/WEB1802 ... Pipe-Elbow