I stumbled on this build from …over 5 years ago. I like how the author used flashing to help heat up more surface area. Not so sure I would go with PEX like he did. There is another article on the site that goes over all the different jigs made to make the wrapper to go around the copper, or pex, pipe.
http://www.builditsolar.com/Experimental/PEXColDHW/Overview.htm