I just made a few tweaks for the bone charm so you can add screws and a nut (M3) for easier assembling.

I made a space for 3 M3 hex screw heads in the main body of the bone charm. And also added a space for a nut in each round mcdooda that attaches to the main body.

I just paused the printer just before the printer would cover the space for the nut in the big round mcdooda and inserted the nut, and then let the printer enclose it.

I think I used 15mm long screws, cant remember.. :)
And glue.