We used to use ones from Fog Hollow motorcycle parts in St. Louis but the same ones are on ebay for less. Be sure to get the on that includes a weld in bung. Here is another shot of one that is painted body color. It is in the upper left corner of the rear apron and you have to look close to see it.
Threaded bung welded in:
[img]https://i.postimg.cc/Bt7Xjqq4/pop-up-gas-cap-004.jpg[/img]

Cap popped up:
[img]https://i.postimg.cc/1nC5VH1J/pop-up-gas-cap-003.jpg[/img]

Cap in down position:
[img]https://i.postimg.cc/0Mb1F1h0/pop-up-gas-cap-001.jpg[/img]

Cap painted body color:
[img]https://i.postimg.cc/1gXd5cM9/two-34-s-outside-001.jpg[/img]


Evan