I lost the nut on the GX20 connector that came with my spindle. I went ahead and soldered the connector to the wires and installed everything.  Although the plug didn't feel like it would pop back out of the spindle, I figured I'd better go ahead and replace the nut.  This is a split nut that can be used for many different purposes (actually adapted from a split nut I designed for a hot tub).  It is parametric so it can be adapted to other uses for split nuts just by modifying the parameters.  

For the GX20 part, I used 2 each of M3x6mm bolt and nut.