Jacques -- QUESTION - do you want this placement to be permanent ?
I have NO problem having it on the roof rack - B-coz. (1) I take 10 liter [5 p/person] inside the car for DRINKING water. This i draw early morning before the Africa sun gets to it.
Then - when I stop lunch time -- less gas / fuel to heat water for koffi [NEVER tea
Afternoon, same advantage -- quicker to get warm water for cooking & washing. That 10 ltre inside the car starts on it's own so you start with 70 litre [that gives you a safe water margin for 7 days / 2 people if you go deeeeeeep in da Africa bush
You can cover your tank with "Ozite Carpet" which also keeps it cool -- And also - it does NOT heat up that much - In Botswana, at 42 deg late afternoon I still drank water from the tank on roof .. Not COOL, but not that bad either [maar dit was anders -- ons was 5 in die kar en daar was nie plek vir nog water in die kar nie en ek was murr dors]