Any gap bigger than 0.1mm can allow enough of the E1 pulse into the container to damage the contents.
You need to have an overlapping contact between the lid and the box.
To be most efficient the box should be made of on of the four best conductors: Silver, copper, gold or aluminum. The least expensive and most available of these conductors is aluminum. The heavy duty foil that is used to wrap turkeys to bake is as good as you need - for the same cost as a galvanized can you can get enough foil to cover a room.