HM's SX3 spline lock does not fit the original Sieg X3 because the motor mount and e-stop are in a different location on the X3. My mill is the Grizzly variant of the Sieg X3, model number G0463

Hackery ahead: I used OpenSCAD to remove the left side of the SX3 STL, shorten the right side leg, and added two ribs to level the remaining part. Printed in Prusa PETG, 6 walls, 15% hexagonal infill - plenty strong to hold the spindle in place. Don't brute-force the spindle bolt and you will be fine.

Attached are four photos, the X3 STL, the OpenSCAD script  (which requires a renamed version of HM's STL), and the STL from the original (renamed to spindle-spline-lock-superx3.stl).