Why do you really need all that extra fuel capacity. The real issue is unreliable fuel gauge.
Rotopaks are tough but for weight distribution would be better strapped to the fuel tank crash bars.
But they are expensive and are they tough enough to withstand a crash at that point on the bike? Also vulnerable to theft.
Money no object -
The battery box has a capacity of 2.5 to 3 litres and sits centrally. How about a fuel bladder or alloy tank made to measure? There is enough space by the ABS under main tank for a Ballistic battery. Ultra low maintenance and high cranking power in a small package.
If the need is just for reserve fuel, one litre will fit in the carbon canister space under fuel tank and plastic 2 litre fuel bottles are under £20 on eBay. Probably tougher than thin aluminium.