As it happens, (and I think it was The Mog that pointed this out initially), I believe that it works very well as a stand alone effect without the cards.
You tell the spectator that you are going to attempt to do some "voodoo" magic, and get them to hold a glass on the palm of one hand and then tell them to cover the top of the glass with the other hand, so that nothing can get in.
You will of course need to position their hand to bring it out in front of them a little bit
You then cover the glass/hands with a large silk and tell them that you will magically make something appear in the glass. With a quick "abracadabra" you whip off the silk to reveal...
oh, a still empty glass. bu**er!
You tell the spectator that it has always worked in the past, apologise and suggest that maybe it will work if the glass is up the other way.
Of course when they turn their hands up the other way it turns out it worked after all, just not in the glass.
(just don't use expensive glasses).
Thanks again to the mog for the general idea...