Dany gets a racing stripe, or how to seal a gas cap?

I fixed my 67 dart cap by buying a mid 60s, 70s ford pickup fuel cap. Drilling out the center and removing the gasket. Picked out all the old gasket from my 67, put it on over the center and haven't leaked a drop since.

My guess the 65 mustang gasket would work just as as well, what I liked about the truck cap is the center hole was smaller than the center of the 67 cap and wedged itself between the center and the springy diaphram

Aren't repos made for the entire cap?