Actually, I had considered a gas cap from a TR-7 sitting in the local junkyard. It is recessed nicely and is about 5" in diameter with six lugs to grasp but no lock. It gives a kind of race car flavor to the opening with its size and style. I haven't quite made up my mind which way to go. It would probably only cost me $5-10 at the Pull Your Part misused car emporium, although I already paid my $40 for the Kawasaki lid.
Jaguar Type 2 XJ-6's and others have a really nice heavily chromed flip up locking gas cap that fits in a recess in the fender and might look quite good on the GT, but the filler pipe would have to be rerouted and there might not be enough slope to the filler for efficient fueling.
Don't force it -- just use a bigger hammer!
The Spitfire filler sits above the body, no real modification and the neck is the same size as the GT's. Just need time to do it.
The XJ6 cap I've looked at also, but It seems too big for the GT and would require cutting the car.
A place called 'Demon Tweeks' in the UK has some wonderful vintage racing thru modern stuff in their catalog (how about slotted drums for a Manta A? Or roll cages for a Kadett?) but, they don't show much vintage stuff on the website.