Could you please describe a "M4" captive nut. (just a basic threaded nut?) I've been trying to come up with a decent mounting technique, and I do believe you are doing would I would like to accomplish. I got the concept...picture would be great if it's more than just a threaded nut. Obviously, it has to be square and true. How do you set it to make it true? (use a 90 degree square and a level?) I've been making decorative cradles with pins through the sides. Although I've never had any problems with this, every time I set the pins, I just cringe. This is a permanent mounting technique, and the pins are hidden. Coming up through the bottom and center of the cradle would be ideal. Your technique would also make the cradle removable if needed.
Guess I could just take a small wing nut that would accommodate a threaded rod and grind it down so that there are some lateral pins for retention and set that in the skeleton?
Anyway, "M4"? Thank you, Ken