I would also be concerned about the fuel sloshing about in a choppy sea, and god help you if it leaked.
I have been aboard a mono hull
yacht (Sigma 33) that had a fuel tank
leak over the winter and the owner wasn't on board. The shallow bilge
meant 20 gallons of diesel went everywhere and soaked into the plywood
bulkheads. I walked away from buying
that one because I didn't think you would ever get rid of the smell.
So what to do? well how about having a custom made tank to fit what space you do have? Okay not cheap
but its a job that you do once and that's it for ever.
Have a look at this, it's a UK company but there must be other in Japan