[ZerosFJ]

Your links don't work for me but there are lots of discussions on aux tanks and other methods of carrying fuel.

Personally I hate the idea of roof mounting, as you say CofG is a big issue as is the hassle of getting it up and down. On the rear door also concerns me due to the thought of being rear ended. I did fit an aux tank, though not huge capacity it makes a massive difference to me and off all the mods I have done in many ways it is the best. If I hadn't managed to get this sorted I would have gone with one of these:

https://www.turtlepac.com

Jerry cans etc take up a lot of space for the volume they hold, a bladder is much more efficient on space. I was going to store under a cargo deck so you retained plenty of space in the rear. I do use a similar set up for 40litres of water in a bag under the raised floor.