The plates that construct the cage must have good inter-connection on the egdes, closing the box. (otherwise the plates could become antennas!)
For professional EMC protection (not protecting the environment for computer RF, but protecting the computer against high-voltage discharges or lightning) the complete machine should be encapsulated inside a metal cage (preferably copper) with only one hole on one side. All cables should enter through that hole and must be filtered at the entrance. All common-mode wires (ground/shield) should be perfectly connected to the cage at that entrance-point.