Well, Diana has an aux. tank, I believe mounted in her side car.
And IIRC, someone else had one mounted on the rear luggage rack.
If you do go this route, just remember that our fuel system is gravity fed, so the tank will need to be mounted higher than the carbs.
That is, unless you use the tank as just a place to hold the fuel, and rig up some way to pump it into the gas tank once it's needed.
AKA: Tim & 'The Adventure Cycle' VROC #24567, NEVROC, SteelCity VROC
"When life throws you curves,
Aim for the apex."