I've had Westies in Saudi Arabia and Morocco (lots of sun). and a black
plastic gerry can will heat up just as well.  I have carried up to 5 or 6 big
gerry cans (side up) in the front roof rack (some gas, some water) and I had
no trouble with weight.  With the gerry can upright in the luggage rack, I've
been able to just syphon the water down for a shower.  For max heating put
the gerry can on its side (during the day) in the luggage rack.

What I'm thinking of (for my impending trip to Egypt) is to use the electrical
feeds for the rear defroster (don't need this in Egypt) and hook up an in-line
water pump and an on/off/pulse switch to pump shower water.  The Egyptian
sun and a drapery over the open rear hatch should do it.