This is a simple part sized to go onto a Shimano-type freehub body underneath a 9sp cassette. The intended thickness is 1.85mm and the model includes a single tooth to engage the splines.

I can create other versions of this if desired - likely you'd want different thicknesses. Realistically, you could make this into spacers to convert a freehub into a single-speed.

I'll include STL, 3MF, F3D files at a minimum. The design was in Fusion 360.

PRINTING

I printed in ABS+, but probably PLA is just fine. Layer height can be pretty tall - 0.3mm.  3 Perimeters recommended. The 1.85 spacer is so small that infill doesn't really come into play.