Been thinking about this as my pool is getting cold already. First build is one I have seen in the past with irrigation pipe. Basically you make a big circle with the stuff. Second build is definately the poor makes solution, using the cooling radiator from a frig. It looks neat, but….little leery of using the cooling parts from a frig in water that may be drunk by accident.
http://www.thesietch.org/projects/solarthermalpanel3/index.htm