Faraday cage - reducing EMI?
Any holes inn a Faraday cage will allow Efields to enter, with most of the E flux to be captured on the INSIDE of the cage right by the hole. Thus is you must have holes, make they look like tubes. And perhaps place a grounded-case capacitor right by the hole, so the grounded-case is a further capture-the-flux metal structure.

Thus if you place a sensitive node, or circuit right by a hole in the Faraday cage, bad things are likely to happen, BUT you can model this. We are assured the behaviors of a finite-element-grid (such as a SPICE array of resistors) is an accurate model of E fields; you can create a grid of resistors, selectively ground some regions, edit Rvalues to indicate metal and air, etc, then run a AC simulation to map the internal fields; you might even build your sensitive  circuit alongside the resistor grid and let the displacement currents directly flow into your circuit nodes.

I will let other folk address magnetic shields.