Hello
I've look at the whole thread this is so cool ! I was wondering if there was any way for you to find a Small Solar cell so you can get your batteries charged . Your Idea of the stiorfoam box will do the trick
A large Plywood box with your black tubing an paint the inside black to will heat your water very well. I did this on a very small scale Raised the water temp. from 60 F to well over 120 F with normal sun shine You could insalate
the plywood box with house insulation of foam any thing that will whole the heat in.. ow well it was just a thought .
Bigdave: