This is a configurable thumbnut design.  The default parameters are set up for a square M3 nut.  This can be configured to fit any size of square nut.

When you're slicing the stl, make sure to set it up to pause at the end of the very last layer where the square opening is still open.  In Cura you can do this by going to “Extensions” → "Postprocessing" → “Modify G-Code” → “Add a script” → “Pause at height” (make sure to set the method to match how you're printing).

I found that configuring it based on direct measurements of my nut didn't allow it to fit properly.  I had to adjust the inside hole to be a bit larger (by more than I expected) before it would fit.  It's really quick to print, though, so it doesn't take long to find the right size.