I have used a filler cap fitting from a Jag XJ (they have 2, which are lockable, oval flip-tops) and mounted it on the wing. It needed some fabrication of a reducer pipe (stainless) as the throat of the XJ item is 2.5" but the tank inlet is 2.0". This then goes through a couple of 90 deg bends to join the original Beetle tank under the rear window. Haven't noticed it being slow, but haven't visited the pumps yet - only filled from a jerry-can. It does lock though so prevents the hooligans with fireworks that live near Alex.
Mick