The tri-arm hub that spins the glass tray. Motor shafts are D-profile — 7 × 5mm on most models, 8 × 6mm on some larger ones. Pull the old coupler off (it lifts straight up) and check the bore; the tray stops turning the moment the D strips.
Match the bore to your shaft, pick a style and color, and get it printed and shipped for
about $20 — or download the STL free and print it yourself.