Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:5649254.

This is a simple threaded blanking plug which can be used to fill an unused hole in an enclosure.

The plug has a hex head which is 19mm across faces and is 5mm thick. The threaded portion is 15mm outside diameter and 9mm length. The nut is 22mm across faces and 5mm thick.

You could combine this with an o-ring or washer for some basic protection against water ingress, bearing in mind the usual limitations of 3D printed materials of course.