reedsterstl, i was just thinking,depending on how big your tank currently is,it would be possible to use an oem tank to make a "buck"
then modify the buck to get the shape and volume you are looking for
then finish the "buck" and use it to make the "mold"
once the mold is finished you wax it a few times with a mold release agent,then apply two coats of gel coat,then handlay one layer of fiberglass matt,and one layer of fiberglass cloth,once it cures it is ready to pull out of the mold.
most fuel tanks require a two piece mold and a special resin that is fuel resistant.
then of course you have the option of carbon fiber and vaccum bagging.
are you still interested? LOL