Water
Well known method to use for shielding places you don't want people sniffing EM from.
Also used for shielding all things nuclear that are hot hot hot.
Can't claim the kill on that problem it was the big Russian speaking bloke who debugs in hex that did it. But we were all in the restaurant that was having the problem with its electronic POS waiter units. The 6'x8'x1' fish tank certainly screwed the signal.