Ambient room humidity...
Perhaps, If you were to put that shoebox inside of another bigger, more or less sealed "sterlite" brand box with some "wet" vermiculite (peat moss...) in it to raise the external ambient "room" humidity,
it will tend to slow down the evaporation rate of the incubator and thereby raise the ambient humidity level in that incubation box. The room wont be sucking the moisture out of the box quite so fast...Good luck with them eggs,
Hope you have a high hatch rate.
imho.