What am I missing?

Not having my Duster in front of me...

Small head sheet metal screws will work and there is a gasket between the metal filer neck and the body. *I think* it is rubber.

Locking gas caps are small in diameter, so, yea, it shows a bit of what is normally hidden. It isn't the prettiest sight.
Yeah I don't like the looks of the locking gas cap because it sticks out pretty far! And now I really don't like it, knowing that its smaller than what they normally are and that they show more! Do you by chance know what that gasket would be called?