A friend and neighbor of mine...an aluminum fabricator (and motorcycle mad scientist of sorts) made dual aux fuel tanks for a local guy with a 2009 Concours-14. I believe it was two 2 gallon tanks...one on each side. He formed and moulded them to kinda blend in with the bike, and form fit the fronts of the side cases. The tanks were mounted on each side of the bike, and bolted to the passenger pegs (passenger pegs were removed). Each tank was plumbed/pumped into the bottom of the C14 stock tank via a bulkhead fitting that was added.
This guy did a cross country (Canada) coast to coast Iron Butt challenge in just under 72 hours...which by the way is much farther than coast to coast USA. If I recall correctly, he was getting 600 km range, or 375 miles, and that was running pretty hard as I understand it;).
I'll see if my friend has any pictures of the custom tanks.
Rem