Look at the 90, loads sold world wide from 80s to present. All owners want more capacity and there is one spot that has not been used that is free on every model from TD/v8 to tdci. The passenger side rear wheel arch!
See this:
 
https://www.aulro.com/afvb/images/im...017/03/628.jpg
Mirror something like that to the left hand side, align the filler so it can be filled from behind the number plate (flip down, very tidy) and place the outlet to the rear.  Great for a water tank!
Here is example of hiding in/outlet with a flip down number plate
https://www.aulro.com/afvb/images/im...017/03/629.jpg
You can see below on the right hand side implementation by equip, they  are not using space towards the top of the arch or flat up against the  outer panels, due to inline placement of the tank between factory filler and main tank. Placed on the left side you can fill the space out  entirely with the filler at the top going out behind number plate.
https://www.aulro.com/afvb/images/im...017/03/630.jpg
As a water tank the outlet would go to a tap on rear cross member. As a fuel tank you run a transfer pumped line across rear cross member to main filler.
 I'll take a water tank please, 😁