Omigosh. Such ignorance being propogated on FR. People, please stop telling people to use old microwaves for Faraday cages for EMP! I'm telling you this as an RF and microwave engineer with over 20 years experience. Just because you think microwave ovens don't "fry whoever is closest" with "radiation" (oh my, your Wifi which operates on the same freqs would be horrified) doesn't mean it's a good shield for the VERY WIDE band of frequencies emitted by an EMP bomb or even a solar event.
Please, just put stuff you want to protect in a closed metal box with a tight-fitting metal lid. No mesh, no open spaces, no nothing.
You are the first person I have heard who has an opinion contrary to our folk lore.
As I understand it a microwave makes a very good faraday cage and simply putting your electronics in a box offers less protection.
In response to your post, I decided to test the hypothesis that radio waves will not travel through the microwave, which is a cage.
I had not done this before but since you are saying don’t:
I just put two cell phones, walkie talkie, radio and a couple of other gadgets in the microwave and not one of them accepted a signal.
If you have information that is more beneficial I and the other posters would be grateful for your help.