Why does my GPS and Wi-Fi work inside a sealed copper clad box?
Even if you have good contact points every cm or so, if your antennas are very near the gaps (a few mm), the RF energy will come inside strongly.

Some months back, I answered a question about "why metal cages around IR receivers".

Why are many IR receivers in metal cages?

Reading Richard Feynmann's lectures, I found the attenuation is 1 neper (8.6dB) * 2 * pi * wavelength/separation.

Thus a 3 mm wire grid (making the Faraday Cage) with the antenna 3mm inside the grid, has attenuation of e^-(6.28) = 1/533 or 54dB.