I don't know if any made it to production but I have talked to a couple of engineers that worked for Olds and they remember working on one for the Fiero. My brother worked in an area where he saw a lot of pre-production parts and he remembers seeing a plastic tank. When I first heard people for Olds talking about it I just brushed it off. I couldn't understand why Olds would be developing parts for a Pontiac. I later learned that Olds did a lot of plastic work for all the other divisions.
Since the Fiero tank is different then any other tank you are going to find it hard to make something else work. Unless you go to some type of fuel cell.
I recently had a conversation with a 'plastics guy' that I have known for decades about a tank for a Fiero. Rotational molding would be the best option, i5 would not be inexpensive due to the internal baffling (doable), but the anti-surge and vent box at the fill point would be problematic.