After my 66 Mustang fuel cap
broke, it sat on my workbench for a while as I pondered what to do with it. Rather than pay $80 for a new one that wouldn't work unless the Mustang logo on the outside was crooked, I decided to try and fix it. I bought a Stant 10640 fuel cap (about $20 at the local O'Reilly's) and installed it on the car. I marked the 12 o'clock and 6 o'clock edges with it properly seated on the filler neck. Then I also marked the inside of the broken Mustang fuel cap to designate the vertical Mustang emblem.

I cut the ears away from the Stant cap with my Dremel, and used my ball-peen hammer to make the inner center of the Mustang fuel cap concave. I test fitted the Stant cap, and pounded the Mustang cap until there was a reasonably tight gap between them. Then I made sure that my alignment marks were properly matched, and tacked the Stant cap to the inside of the Mustang cap (it took a couple of welds before I realized that I needed to protect the rubber flange from heat, and I removed the cable because it was getting in the way).
So now my Mustang gas cap is functional and when it's tight, the Mustang logo is properly oriented. As a side benefit, if I'm feeling contrary, I can put the cap on so the Mustang emblem is upside-down (couldn't do that before because of the large tang on the original cap).
The welds are tacky, it looks awful and needs to be cleaned up, but it works.

Note that the Stant vent only allows air in so there may be pressure inside the tank when you remove the cap for fueling. Also, when you're welding the parts together, be sure to do small tack welds and let the parts cool between starting new welds so you don't compromise the rubber/plastic seal.