But what I thought would always be cool, would be to get an military
surplus aircraft fuel cell bladder, that lays right in the keel of the
boat. they deflate as fuel is used. ...

Beware of chafe problems with them; they've been known to rupture. Of
course, you could tow the bladder behind the boat You'll want to
put a diamond day shape on the back of it and an all around white light
at night.

-- Tom.