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.
“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.”
No, that’s not correct. The metal container acts as a Faraday cage which do not allow the electromagnetic pulse radiated signal to penetrate. What is important is that it is a good conductor (like aluminum) and that there are no openings or gaps which can act as slot antennas and allow EMP penetration. Everyone knows that if you’re in a fringe cellular area, you’ll get a better signal if you get out of your car - that’s because the car is acting as a poor partial Faraday cage and only permitting microwave signals from the cell tower to enter via the glass areas. Think of an armored car and you’ll be much closer to a good EMP shield and be unable to get a cellphone signal at all.
For the lazy and/or paranoid who want to protect their small devices against the chance of an EMP while they’re out shopping or whatever, just put them in your microwave oven (don’t turn it on!) Microwave ovens have to be very effective Faraday cages to contain the high power microwave energy that would otherwise cook or neuter you, and they work equally well at keeping electromagnetic radiation OUT as IN.
So just build yourself a giant microwave oven-like room. They’re all over industrial labs, known as “screen rooms”. Screen is effective as long as the openings are small enough that energy of the frequency of interest cannot penetrate.