This heading of this ring is a disc with 3 curved/straight slots.
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. On model 8.2 the ring is red - similar to the heading.
The ring is available at the following marketplaces.