Another idea: you don't need a mirror glass. Cut two pieces of Mylar slightly larger than a playing card and glue them on to a thin piece of card board. This goes around the inside of the glass, not in the middle, but pushed against one side so the cardboard curves and is right up tight against the side. Put the dupe card behind this.
Have a an 18" square of black silk bunched on the table. Lay yhe glass on its side, mouth towards you, mirror side down. This looks perfectly like an empty glass.
After the dupe is forced, pick up the glass and silk, hiding the mirror side against your palm. Hand the silk to two spectators and have them hold it out by the corners, like a trampoline, and drop the deck in the middle of it. Have them toss the deck up in the hair, etc.
Another idea: a cone of plastic that fits the inside of the glass with the Mylar on one side. This can be stolen out if necessary.
I have that Emerson trick somewhere. I think there was a secret pocket in the hank.