An insulated metal box within another metal box which is properly earthed should be fine to protect sensitive electronic equipment against most forms of naturally occurring EMP. i.e. say a laptop needs protecting then a aluminium laptop case inside a aluminium storage chest inside your earthed metal shed should work out fine to protect the laptop against EMP. If this doesn't work against a very specific man made EMP scenario then chances are your not gonna be around within the next 10-15 minutes for find out if it didn't work anyway...