Don't use kitchen appliances for faraday cages. Use faraday cages for faraday cages. Either buy one or make one. If you're willing to spend the money a microwave oven costs, then spend the money on a faraday cage/box/bag instead.
To make a good faraday cage that can block electromagnetic radiation from EMP's and CME's you need serious shielding. Microwaves aren't built for EMP's or CME's in mind. This is sufficient reason for not relying on a microwave oven.
You also need to ground the faraday cage. I can ask you, how are you going to make sure your microwave oven is grounded well enough?
Make or order a box made of copper or aluminum. Place wood, plastic or other non-conducive material inside along the surfaces (so the electronics aren't damaged by a surge). Make sure it's grounded properly. Also protect it from moist, water, mould etc. (for the electronics you're trying to protect). Use plenty of faraday tape around the lid.
I've seen videos where people make their own faraday cage using a cabinet padded with foil, and then some faraday tape. Most likely won't work!
Against the extreme radiation of EMP's/CME's, you need extreme shielding. Optimally, you need METERS of LEAD or SILVER to protect against an CME, and there would still be no guarantee it would be enough.
Which is why I use EMP Shield. My entire properties electrical system is protected as well as everything plugged into it.