These are Endcaps for the legs of the Ikea Lack Side Table, popular for use as Printer Stands and Enclosures. One table at 17 inches is too low, and two stacked tables was too high. So I cut the legs in half, filled them with foam (the Legs are hollow) and put these Caps on them.

Originally I was going to use Machine Leveling Feet so the design accommodates a 12mm T-Nut. I eventually went with self-adhesive non-slip pads. If you choose this option, there's a file with .5, 1, 2 and 3mm Shims to balance out a short leg.

BTW: I increased the Infill from the default 20% to 40% for a little more support.