We had a lamp post with the lid off. So I printed this key, after fixing it, to put in place again.

It's not a constant width polygon, so the sides didn't match. So I sketched over a reference photo with 2 arcs with FreeCad.

It perfectly fitted on first try. I have given 0,2 mm of extra margin on top of the height of the triangle. It's a modificable constraint of the sketch.

Prints fine without supports.

Dirt inside from using it in first photo.