I sometimes use old quarts of oil bottles to carry a little extra fuel and stick them in my backpack. It's hard to find a bottle that does not leak and usually you can only use them only once.
But I do have two motorcycle tanks that I will swap out with the small tank when I know I am really going a long ride. One is a gallon and a half and the other is two gallons, I prefer the 1 1/2 gallon cause it's easier to pedal. The 2 gallon is a little wide and my knees hits it once in a while.
I have also seen many that will carry an extra gallon or two of fuel in a gas can and they will use a rack, or front basket or something to carry the fuel cans.