I met two Hungarians in Morocco this spring, who had devised a simple method for carrying extra fuel. They took two cheap plastic 5 litre fuel cans (like you can buy from most petrol stations in the UK) and rested one on each pillion footrest. Each can was held in place with a canvas strap attached to the neighbouring aluminium pannier. Effectively the footrest took the weight, and the strap stopped them falling off the bike.
This seems like a good idea because it is cheap and quick to implement, the weight is kept low, the cans are protected during a crash, and they don't intrude on the rider's space.