if it is to protect them from the EMP from a nuke, or other world ending events, then it is useless, your ledgers will have no value without internet, and your other data will only have value if you survive and you have something to read them, if it is to protect it from your own high voltage experiments, i would put the experiment in the cage, not the drives etc. but either way a big metal box is a Faraday cage, if you want to make it out of mesh the thinner the holes in the mesh the higher the frequency it will be able to block fully, for a tesla coil you would want rather thin stuff, you want smaller than the wavelength you want to block optimally, that is if i remember right, those lessons were a long time ago and never used since then
given what you want to put inside, an old computer case with no glass panels would make for a decent faraday cage, it is already meant to keep the EM interferences from the inside of the case from interfering with things outside. you can plug the psu hole with aluminum tape to make it extra safe if you want.
