There are countless aftermarket tanks available in just about any
configuration and size you could ever need.   I looked, first for
competition fuel cells when I built my Porsche racer and again when I
needed to replace a water tank in my Alaskan Camper...     Really, you can
find almost any size and shape with inlets and outlets just about
everywhere you could imagine or ever need...Find a space you want to put
the spare tank...measure it up and go looking.   You can plumb the spare
tank to automatically feed into your main tank, or have it switched.   An
in-tank feeder pump for the secondary tank, and you are set.   Any of the
racer supply houses online have ooodles of filler options or you could
branch it off the main filler neck with some thought.    There are even
'soft' fuel bladders that will conform to odd shapes...used in yachts.
It's simply a matter of thinking it all through, finding the proper size
and configuration and doing it...

here's a link to get you started..
http://www.atlfuelcells.com/?gclid=CjwKEAjwl7ieBRCK2rCtqcCS7jESJACZKQFKzjHbx3J2yxhwuw3svN1SovK03HBBiOCxKUVGzqDsIRoCpsjw_wcB

   When I did my racecar, we put narrow but deep orange juice pitcher
upright inside the fuel cell....then we plumbed two feeder pumps, one from
either corner of the fuel cell to feed into this OJ pitcher, where the main
fuel pump picked it up and supplied the 6.5 liter beast...That way I could
run just as little fuel as I needed to go the race distance without having
fuel starvation issues on a long high G corner with a fuel pick up being
uncovered...either or both feeder pumps supplied the OJ pitcher and kept it
full and overflowing back into the cell always....except when I miss
calculated and ran completely out of gas.