This should be a close fit in a 1.25" eyepiece adapter like this one:
Check your printer calibration with test_cheshire.stl (only 5 mm high), eventually modify the diameter in the openscad file and regenerate a stl.
Print collimator_grid.pdf or dxf, cut, center and glue on the inner bevel. Enjoy!
Also included a mask to mark the center of a 4.5" primary mirror.