I had used some 19x13mm OD Aluminum extrusion to make a headphone holder for my desk out of an old yard umbrella. (The aluminum from those is super handy)
It had a plastic cap but it was smooth and sometimes they would fall off, so I designed this.

It was actually designed before I got my printer and it was the first thing I printed, as such it didn't adhere to the bed well, but the part you cant see inside the extrusion printed perfectly.