My '93 +8 is the same. I've often wondered how to do what you are suggesting. Short of re-panelling the whole back panel, the only sensible way I thought of was to make a smart looking stainless panel with a round hole in it for the new filler cap, that covers the oblong aperture. You would have to fix it with nice looking screws or rivets and probably edge it with some plastic trim or small beading. It needs to look as it was always meant to be there, which might be tricky, which is one reason I have never got round to doing it.
