It’s a metal box, or mesh box, designed to protect electronic components from nuclear EMP and solar flare type stuff. I’m using cookie tins :) you have to keep the contents from touching the sides/insulate them, but tin foil can work in a pinch, just not as well.
http://en.wikipedia.org/wiki/Faraday_cage
What do you use to insulate with, foam padding? And can you use any type of metal box?
Ammo cans work well... but you have to remove the rubber seal “usually” and replace it with mesh... although I read an article about electrically connecting the top of the can to the bottom without sacrificing the seal. I haven’t tried it though. I’ve also heard of “stuff” that can replace the gasket AND provide an electrically conductive path as well...but so far have not seen it.