Just something simple: Put the 10/6 gaff atop the face-down deck. Top it with a face-down normal 10S. Put the normal 6D in mid deck and force it, e.g. by cutting to a break above it. Have them take and memorize it, perhaps showing it to others but not letting you know what it is, while you set up a new break under the 10/6 gaff. Let them keep the 6D or return it to mid deck, but tell them you'll have the deck magically identify their card for them. Tap the top of the deck, and single lift the normal 10S. Tell them you'll command the deck to reveal their card ON THIS very card. Flip it back down, do your favorite mumbo jumbo, finger snap, cover with a handkerchief, blow on the deck or whatever, then DL to reveal the gaff, which reveals their card actually ON this very card, as promised.