Its been a cool summer in the North East, so my thoughts have been drifting lately towards building one of these again. Only thing I would do different is use something else besides outdoor zip ties (maybe wire or something cheaper).
http://www.instructables.com/id/Overview-2/
Another interesting version is here that maybe worth doing since its more “removable”.
http://www.instructables.com/id/Cheap-solar-hot-tubspapool-heater/