[Greenbuilding] Batch Water Heater Tanks?

[wmdorsett at sbcglobal.net]

An old friend who has little money wants me to build a batch solar water 
heater for her using the ol' tank in the refrigerator trick. You plumb 
it in series with the regular water heater, face it to the sun and put a 
piece of glass over the door opening. On extra cold nights, you close 
the door preserving the heat.

The question is what do you ask for to get an uninsulated 40 gal. glass 
lined tank? For an upgrade on the tank, what do all of you recommend as 
a non-corrodible tank for this application? It has to handle line pressure.

Thanks for your thoughts

Bill Dorsett
Sunwrights
Manhattan, KS