Do you want it in the same location as the original factory cars? If so, scale from photos; make the hole then locate fuel filler on the tank accordingly.
If you are using a VW fuel tank, the filler neck attaches where the stock fuel level sending unit is located, in the recessed center of the tank. Dry fit your tank & filler neck; use the neck to mark the underside of the hood; start small and enlarge as needed.