You borrow a coin, you ask a spectator to sign it. Then you make the coin vanishing with your favourite method.
Then you make a circular metal box closed with three rubber bands appearing and you hang it to a spectator.
You ask the spectator to remove the rubber band and to open the box. He will find another box also closed with three rubber bands
Once again he opens the box and discover a bag in which he will find
the signed coin!
Delivered complete with the rubber bands and the holder!!!