This heading of this ring is a simple rectangular box with an opening in the middle to receive the ring.
The heading is laser cut from 4mm birch plywood board. The ring itself is 6mm thick.
The size of the ring itself is selectable, available in 18 sizes from 15.3mm through 22.3mm inner diameter (US 4½ - US 13). The ring simply sticks into the middle of the heading. Sides of the ring is untreated, the outer curved surface is painted to black.
The ring is available at the following marketplaces.