The tank is original to the plane as far as I know. I am at least the second person to fly it, and it had a few other owners too. I think it is a Perfect wedge... but the tank was mashed a little to fit the space.
I would make a pinched corner type using some gallon olive oil tin can, slightly thicker than .008, more like .015 but I have a pile of it. Just needs an acetone wash to remove any remaining residues.
http://aeromaniacs.com/Tips.htmlthe George Aldrich uniflow at the bottom, but probably not uniflow in this case.
I have some rather large balloons that hold close to 3 oz of fuel without stretching, they are called "large party balloons" from the grocery store. This is why I wanted to know about a Fox on a non-pressure bladder.
Phil