Way back when, when I was going to install an aux. tank, I was going to try and slip it in under the luggage rack and where the tail light and license plate goes. Basically an add-on vessel made to fit the bike and carry the tail light, license plate after doing my best to 'de- uglify' it sufficiently. I think three gallons would sneak in there pretty easily but 5 gal. could be done as well. It would take at least one fuel pump to make it work (transfer the fuel from the aux. tank to the main tank) but I was planning on installing two so I could fill the aux. tank from the main tank without getting off the bike and fooling around with a second tank, plus that eliminates any outside fill caps, vents or anything else. I ran a line into and through the fuel tank that terminates just above the splash plate to serve as a vent for an aux. tank.
I got as far as the rough design, the plumbing in the tank, and the valving but never procured the material or did any of the fabrication.
That flat plastic box tank is a LOT less work and will still save one's bacon out in the sticks though and probably the better idea.
Brian
I'm thinking a semi "stealth" install possibly with a pump to transfer the fuel.
Of course the whole lot would need to be bolted down properly so as to avoid wobble