I would purchase the rubber seal that goes around the tube and screws to the trunk floor. It has the proper shape needed formed into it.
Trial fit the tank and tube after cutting a smaller oval shaped hole to fit the filler tube to mark the final location. Mark where the seal and screws will mount, then cut out the rest.
I made a small hole for the cuda filler, then decided that if I left it small, water would sit on the remaining lip if I didn't cut it all the way out. You'll see this when you go to screw down the rubber seal.