Have you ever seen a Mini? Where else would you put it? 

Actually it wasn't uncommon to have the battery under the seat. Dad's 38 Dodge had it there.
Now, my 75 FIAT X1/9 had the gas tank behind the driver, the spare tire behind the passenger, and the battery under the front "bonnet" (boot lid) even though the engine was in the middle, just in front of the rear trunk, which was big enough to hold a coupe of 24's of beer, so could have easily contained the battery. (Never hauled beer there though - too warm, being over the muffler.)

...for almost the same reason why they thought it would be a good idea to place the battery under the rear seat??

Why would you want to put a container filled with a highly flammable substance in the interior of the mini?

No reason why you couldn't have a custom tank made. Lots of places around that would do a good job. You just need to figure out the dimensions and come up with a drawing of what you need. This is one place that could do it, I'm sure you could find others. https://fueltankparts.com/pages/custom-fuel-tanks

Don has/had one...

https://www.facebook.com/commerce/listing/261544712262145/?ref=share_attachment

I know that getting your hands on an internal tank from a very early Traveller is like finding hen's teeth, BUT apart from the occasional find on the web, has anybody anywhere attempted to replicate that same tank? Maybe something like the water tank for Campervans but made out of metal? Would that be something doable? Just wondering,,
cheers,
Abel