The classic home EMP-proof container is the metal garbage can. Line it completely with cardboard or thick paper and after closing seal it with metallic tape or similar.
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duct tape wouldn’t do?
I doubt it, although I have only read about that. You are not making a weather seal; you are sealing out a magnetic field; the lids on those containers are not usually that tight fitting. That is also why the inside must be lined with something non-conductive. If the contents inside are in contact with the metal walls they are subject to the EMP.
My military case has a 'tongue' on the edges of the lid and a groove on the edges of its base. When closed there are four clamps on each side guaranteeing a snug tight seal. These cases are available around. We have a general surplus store here that usually has them.