The card is vulnerable every time you remove it from the shielded enclosure to use it so why not just disable the RFID transponder system on the card and forget about shielding it? On my card I can easily identify the chip and it's antenna traces by shining a bright light through it. I can mark the location with a pen, then discharge a high voltage arc through it from a pair of electrodes, one on each side of the die. An expedient method would be to use the ignition coil lead on a car while cranking the starter or while it's running. Zap, zap, zap and it's done.