I do long distance travel and some extra tank w/o any mods was missing on the market, so I decided to do one.
It fits to 950 or 990 Adventure without ABS
and the only mod you need to do, is cut the underseat box and link the underseat tank to the main tanks. It has petcock, so you can close it and keep it as a reserve.
You fit there 5,3 liters of petrol.
I can produce some, if anyone is interested.