Summary

These are the pieces I use for creating the neck joint on my LPS models. They are in .stl format, but they can be easily imported any 3D modelling program.
Instructions:
(ball joint file) Align the bottom of the ball joint with the top of the body, embedding it slightly. Then do a union to join the two pieces together.
(ball joint hole file) Align the bottom of the ball joint hole with the bottom of the head, then do a Boolean > Difference (or your program's equivalent) to cut out the hole.
Have fun!