A Faraday cage does not need to be grounded. It's essentially a box of conductive material.

I’ve made them of wire mesh (hardware cloth, 1/4" (6.35mm) to block Wifi (2.4 GHz, wavelength of 125mm, 5GHZ, wavelength of 60mm). The holes in the mesh need to be smaller than the wavelength by a factor of (not sure, but 10 seems plenty).

The ‘shortest’ mobile frequency seems to be 1900MHz/158mm, the longest 380MHz/789mm, so it works for that, too.

But this is all about mesh, with openings. A solid conductor will block pretty much anything — and it’s not clear what the side-band frequencies leaked by devices (and so picked up by van Eck phreak devices) are.

FWIW, I got interested in this after reading about it in Neal Stephenson’s Cryptonomicon, which might be my favorite novel, ever.