Most practical and organic Signed Card to box that you will USE.
Imagine able to show the box of match completely empty before anything happened. Spectator then holds onto the box, you never touch it anymore. They will be shocked when opening the box by themself again and find out their signed card was magically appeared inside there.
Organic and everyday object, everyone knows matches box
Anywhere anytime, full worker routine taught in intricate detail
Less than a minute to construct, easy to perform