People use the grate inside the boxes in the 'substrate-less' method of incubation, where the eggs don't actually come in contact with the incubation medium, but rather sit on top of the grate that's over the medium.
Using this method allows you to completely soak the medium(vermiculite/perlite or whatever you choose) with water and virtually never have to worry about humidity going down, or the eggs getting wet since they never actually touch the substrate.
Egg crate/Light Diffuser can be found at hardware store like Home Depot
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