I needed to print a hook to hold stuff from the picture rail in my room. This hook should work for others, but I was pleased with how I made the profile fit correctly, listed in the instructions.

Take a picture of the side of the picture rail (my room has an internal corner, so it was easy.)

Draw a hook around the picture rail in inkscape, and export the dxf.

Extrude the dxf with openscad, and use difference() to cut two holes in it for the thread.

Export, print, etc.