We needed a gas cap for our mower so I I modeled one up over a few days. It has interesting threads so I had to figure out how to spiral a custom face which was interesting. I suggest printing the "Final" version but the v2 works just fine, it just isn't as accurate because I didn't know how to properly sweep faces yet. Final is the most accurate but v2 feels better to screw down because of the huge grips.
PLA might breakdown over time as a gas cap so a gasket of some sort might be useful.
I added some tinfoil to mine just to stop it from splashing up and onto the plastic as much.
Made in Fusion 360 based off a side profile picture.