This handy little piece turns a regular M3 nut into a T-nut that fits into a 2020 / 2040 extrusion ( e.g. Ender 3 frame ) in order to attach anything.

Some pictures show an original Ender 3 M4 T-nut for comparison.

Prints best with no supports on one of the large flat surfaces. 0.16mm gave reasonable results.