This is a cap for male quick connect hose adapters. I have these adapters on my soaker hoses that are permanently buried in the garden, so I put these caps on in the winter to keep cruft out. I've tried it on several different generic brands, and it works as expected. You can find a file by Baratus114 for the male hose adapters itself.
The walls of the cylinder are 1.2mm, so it plays nicely with a .4mm nozzle. It's designed to be printed without supports. I printed fast and sloppy at .3mm layer height, and it works just fine. I'm not sure how well PLA will hold up outside in the freezing weather, but they're so cheap I'll just print em again next fall.