The most critical components of a Faraday cage are continuous conductive material around whatever you're putting inside it and an insulation layer (a layer of cardboard will do, plastic or air works well too). There's a school of thought that suggests another layer of continuous conductive material on the outside, insulated from the inner layer. Any devices being protected should not touch any of the conductive material.

A large steel toolbox (often called a "job box") could work as the outer layer, although I'd be careful to make sure that the lid was tightly fitted and well-connected electrically to the main body.

My expectation is that something like a Carrington Event would damage or destroy large electrical grid transformers, along with anything else connected to long conductive wires. Smaller grid transformers connected to shorter wires may or may not be affected. Based on what I've read -- I'm certainly not an expert -- complex electronics within cars and small devices are more likely to survive than not.

Certainly, a close, strong EMP would damage or destroy almost any electronic device.