Hey, welcome to the forum.
The only custom gas cap I can ever remember seeing was fabricated by Gary "Shock Bushing" Eckermann (aka Þrúðr). You can read a little about it
here. Unfortunately, Gary passed away last year. If you search for posts by "Þrúðr" on the Old Tech Archive, you might find more information.
Other than that custom job, I do not recall seeing any non-OEM replacement caps.

Good luck, and again, WELCOME.