I have recently spent more time on EMP research and testing and have come to several conclusions. The way I am now testing my Faraday cages is by placing a cell phone inside and seeing if it will ring. This has lead to some interesting results. The first thing I found was that the use of a microwave oven for a Faraday cage is very questionable. They will not block a cell phone transmission.
A 50 cal military ammo can lined with styrofoam with the rubber seal still intact was successful in blocking cell transmissions. I have always been told to remove the seal or it won’t work. Another thing that I tried was a tin can with a friction type lid lined with a non-conductive material, similar to the cans you buy popped popcorn in. These worked well. You may want to sand the inside of the lip of the lid to make sure you get good contact.