Thanks. My first worry is the loose small pieces disappearing, rather than making the arm fit perfectly. Would you keep those in separate small containers inside the box? A fitted home for every screw might be a bit much.

Yes. Label the containers by what belongs there. Save the fitted shapes for something that needs protecting.

A contents picture inside the lid might earn its space. Labelled tubs tell you where to put a piece; the picture could also show which tub has gone missing. I would start there before making an insert.