Many years ago I worked for a company that made flexible tanks and I have installed them in a couple of boats including one I once owned.  The tanks I have installed were used for water and sewage. I have seen flexible tanks used for fuel, but I have no direct experience using them for fuel.  Some of the barge companies use them (huge ones) to transport fuel to remote Alaska locations.

They're the easy answer for many installations.  Keep in mind that a flexible tank will chafe as the fluid inside sloshes around so securing the tank is very important.  It is also a good practice to pad the area where you place the tank to help minimize chafing.  I used 1/4" closed cell foam for padding. 

The tanks (regardless of maker) never seem to hold as much as they're listed capacity...  Installing the fittings correctly into the tank is critical to eliminate leakage. 

My 2 cents.

Kevin 

John,

If you want a small aluminum tank that will easily fir in the area, then make a cardboard mockup of the tank and slip it in to make sure it does fit.  Be sure to locate the deck fill to correspond to any fill plate already on the deck.  Use the mockup to make a drawing of the tank.  If you send the drawing to me, I can get a quite on making it, or you to can take the mockup to a local aluminum fabricator and have them make the tank for you..