This heading of this ring is very simple: a single circular hole inside the flat square surface.
The 15x15mm heading is laser cut from 4mm birch plywood board. The laser cut faceted ring itself is 15mm thick with outside painted to black.
The size of the ring itself is selectable, available in 8 sizes from 15.7mm through 21.5mm inner diameter (US 5 - US 12). The heading is glued on the ring.
This ring is available at the following marketplaces.