Aluminium is a good conductor (though not as good as copper) and a box made of this material will serve, to an extent, as a faraday cage.

The effectivness of the shielding depends largely on how well made the box is, and how closely the lid fits.

Additional protection may be obtained by placeing equipment in smaller metal boxes, within the larger box.

The most likely cause of an EMP event is the detonation of a nuclear weapon, and a well made metal box although not giving total protection, should protect against anything that the owner of the equipment can survive.