Five Keys But ONLY One Unlocks the 5th Dimension
Display 5 keys each with a different colored tab ring. Explain the famous 5th Dimension key is one of the five that does all the magic! Just have ANY random audience member each select a color key.
This leaves just one key left for the magician. Remember only the 5th key takes the magic to the 5th Dimension which unlocks the lock.
So everyone is positive of their selection, they may exchange keys and even switch for the 5th key that was left over for you. Finally everyone has one different colored tab key.
The magician opens the lock with the key that was not selected!