I lost the nut for the VOSS.farming 561877 solar panel mount, so this is an improvised replacement without researching how the original looks. 
It is made for hand tightening, so it does not adhere to standard sizes.

It should also be useable for some other ball joint fastener screws, who knows:

• Pitch: 1.5mm
• Major Diameter: ~ 20.1mm
• Minor Diameter: ~ 18.8mm
• Thread length: ~ 7mm

The OpenSCAD-File with some parameters is attached, so you should be able to adapt it for other dimensions, as well. It's not very advanced, you just need to install/include “Chamfers-for-OpenSCAD” (see credits) for it to work. Rendering may take a bit of time and memory.

Print:
I've printed it in 0.1mm layer height, 20% infill and 3 perimeters and it works without supports. There are some tolerances built in, but the original thread isn't the deepest, so make sure your printer is more or less calibrated well.
 

Credits: 
I just tested OpenSCAD with this, so obviously I barely knew what I was doing.  The following projects basically did all the work:

• Threads based on “Parametric threads in OpenSCAD” by @makouski_2412202
• Chamfers from https://github.com/SebiTimeWaster/Chamfers-for-OpenSCAD