Why not get one custom made, the only difficult item to find is the looped heat exchange tube, but most good pipe benders could make up something close that would require minimal welding and then just get a fabricator to weld up a box of the size you want.
We used a stainless steel
body and blank ends to form the basis of our central heating
water source. Slightly different to your needs as it uses 240 regular house hold water heating
elements but you get the idea.