This is a mobile installation and the space on top of the van is limited. Skylights scattered around the roof awning at the side tv and satellite dish are all strategically place leaving a 2000x1000mm space for a 400W mono panel. There is no metal in the water heating setup so no galvanic corrosion dangers. The PP panel will be stuck to the back with a thin coat of glue and 4 wood crossmembers distributed along the back of the panel to reduce vibrations and protecting the the glass on the road. I've decided that the thickness of the corrugated PP panel will be 2,5mm so, the total amount of water in the "heat exchanger" will be around 2.5 liters. The circulation pump is a 12v brushless type of motor capable of 8 liters per minute so, lets see how it goes. If it does't work, All IO have to do is stop the circulation and get another heat source. Maybe a couple of turns of flattened copper pipe wound around a small length of exhaust will add a little heat also, so, can't know for sure unless you try.
These options are so cheap to implement, and if nothing comes from it, at least you learn something.