After some tests, I found out that the following trick helps to keep air humidity around eggs OK during storage and incubation. Eggs are often shipped and stored in small plastic bags with some peat. To avoid these drying out, I store them in a plastic container together with some additional bags as in the photo. The extra bags contain a pluck of filter material, which I mildly wet. The water seems to pass through the plastic to keep the peat and eggs in the other bags at an appropriate humidity.