Replacement gas can cap for 5 gallon, 3 gallon and 1 gallon cans that use a tab on the spout and rim on the cap to hold the cap on. They're usually yellow plastic. So far has "just fit" three different cans I've used it on.

Yes

Yes

.4mm

100%

Lots of walls should make it very strong, as close to 100% infill as you can. Print it with the rim down and just punch a pair of needle nose pliers through the rafts to tear the support away. PETG seems to hold up ok to gas fumes.