I did fender mount using an AD cab filler neck grommet, I just cut a hole then turned the hole edge up a bit with pliers, little at a time. Nice new soft aftermarket grommet fit in and conformed to fender shape easily.
After i did it and bought a locking cap I wished I had put it lower on the fender, the way it is the key slot is facing up and any rain on the slot will get into the tank. That cap you posted is cool, I wonder if it would drain outwards or inwards.
The square gas flap posted above looks better than I thought one would, I decided against a flap thinking it would look out of place.
