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I'm not a physicist, but doesn't the conductor need to fully encircle the card to make a Faraday cage? I would think that something U-shaped (in one dimension or another) like a typical wallet would just act as a waveguide. Even if it does act as a waveguide, it might make it safe enough. There might be a way to put some sort of flap that closes over the card.

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There's also one other important requirement: it must be GROUNDED. So unless you're willing to wear a wire from your wallet down to the bottom of your shoe, this won't be too effective.

No, so long as it is electrically completely enclosed, signals won't get in or out. Doesn't matter if it is floating at +/- 13.89 v ac/dc above ground.

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