Card under the box seems like an actual effect and more like the climax of an actual routine. What with how you could literally end just about ANY card routine with it. ACR ends with card under box, etc etc.
I would say that any opener should show the group that you know your stuff and be quick and to the point. Something simple like Card in hand would work, or simply even simpler by just having somebody select a card, control it to the top and to a DL, then do some transformation into their card or reveal it however you see fit. In your pocket, wallet, reversed in the deck, in your shoe, in a card box, under card box, etc etc.
Really it's pretty much up to you and it also depends on the situation you are performing in.