Thats still loads of money, do you honestly need a bigger tank?
We have two 2lt fuel bottles in cages on the ally boxes and a 5lt fuel can held in a custom bracket that sits on the pillion foot peg. Gives us about 24lts and a 320 mile range. We have the option of leaving the cans empty when not needed and having fuel in the 2lt bottles makes it easy to fill the stove.
Will post some pics later.