A blank for a round shift knob. It's sized for a Caterham or Miata (same transmission). Will need to be tapped with an appropriate $4 tap from your favorite retailer; some MX-5 use 10x1.25, my 7 uses 10x1.5.

Two parts, they print with no supports and will snug together when installed to keep it from rotating. For best feel, chuck it (while tapping) and spin the parts in sand paper.

Designed to be leather-wrapped, but the (sanded) plastic feels good too.

Note that the shifter can get pretty hot in warm weather, so print in TPU, PETG, PC, PA, or something tolerant of heat.