I have a choice of two auxiliary fuel tanks. Comes from doing slightly crazy mileage, like in multi-day rallies. The "scavenger hunt" competitive kind, not the "drunk weekend in the park" type.
Anyway ... installed a bulkhead fitting on the tank, next to the fuel pump. Several feet of rubber hose from there to a mechanical valve usually stored under the seat. Well, not *ordinary* rubber hose, but the braided steel outer shell business. Leftover kit from the LSR bike.
Simple setup. Hose clamps, more rubber hose mounts to the tank. One is a "beer keg" shaped AL 3.5 gallon rig mounted to a repurposed bit of hightech plastic to give it a flat mounting surface. The whole rig goes on the pillion with two pair of decent quality straps, black rubber sheet covering the seat. The rubber stuff for workshop floors. Protects and cushions and all that. Tank also has a vent right at the filler cap, more rubber hose connects to a vented catch can. Another valve on the tank itself means the tank can be pulled without leaks even if full.
Second setup is Lower valve is closed when the tank is off the bike, always on if the tank is installed. Tank valve controls gravity feed into the main tank. I ride 120-150 miles, then reach back and open the valve and the 3.5 gallons trickles down, filling the main tank. Fills rather slowly, takes 10-15 minutes for the full transfer.
Everything was chosen for simple, reliable, low tech operation. Works great.
The other tank is 5.0 gallon plastic square that replaces the top box. I can run solo with top box and 9.5 gallons ... or two-up with 11.0 gallons and luggage strapped to the aux tank. OR solo, both tanks, and 14.5 gallons. Trying to figure out WHY I would need 600 miles nonstop range to reserve.
After 300 or so I usually need a bathroom. Yes, I have drained the 9.5 gallon setup from full.
saxman