One of the key concepts to realize with 'van Eck phreaking' is that no shielding provides infinite attenuation at all frequencies. Even solid copper shielding has a finite, if very large, attenuation. With a cryogenic-cooled HEMT or similar front-end and a high gain antenna, the requirements for shielding could be as high as an attenuation of 100dB or more (copper screen is good for 30dB or so typically).
A cryo HEMT front-end isn't that far out of reach, even on pennies, as dry ice can get the temps low enough to foil thin shielding, and thicker shielding can be defeated with liquid nitrogen temps. Specialized near-field antennas that work on magnetic induction principles foil even the thickest pure copper, tin, or aluminum shielding; you need a ferromagnetic shield (mu metal is good) in addition to the copper to shield then.
Vent holes are the hardest, as you then want copper honeycomb material to act as 'waveguide beyond cutoff' attenuators. Slots and gaps of any kind can act as antennas; the Parkes radio telescope, for instance, has a webcam that required a very special enclosure where even the screw spacing had to be controlled. (see http://www.atnf.csiro.au/outre... for details).