Tiny Coupler (8mm to 5mm)

Tiny coupler for z-axis. Most other designs weren't slim enough and the 3mm screw
clashed with the linear rod.

Link to onshape file:
https://cad.onshape.com/documents/2f0a92650016158a002b6834/w/1728f40ec1e0ace04b4d97d9/e/f604a1e68207087ef4244848