Calculate field strength for shielded isotropic transmitter
You must research into Shielding Effectiveness, which is quite a broad topic itself.

As a starting point, take a look at this 2-part article about shielding of boxes and enclosures by Michel Mardiguian: part 1, part 2. 

It takes into account both skin effects and reradiation due to apertures, which is probably what you need: