Glue in an M4 nut in the middle of the back of the "body ball". One of the battery magnets will later attach to it? on the outside.
As a last step before packaging as a kit, I add two sticky pads (foam tape, with the top cover layer kept on), where the arms are to be attached to each “body-ball”. As many types of glue do not stick well to the balls and neither does the double sided tape, I glued the pads in place with hot melt glue. An alternative is contact glue.