The finished piece was a bit of a disaster. I think it LOOKS great, but the hinge is quite wobbly and the screw thread doesn't hold well (I don't know if I used the wrong combination of tap & die in the first place, or have stripped the thread during buffing & polishing).
The safety chain has also been troublesome. It can't be soldered to the screw because it would just get tangled round & round. So I attached the chain to a tag with a hole bigger than the screw thread, allowing it to spin freely.
The problem is how to keep this tag attached to the screw. I attempted this by clamping a small washer shape around the screw thread. It doesn't really work.