A tip to prevent the light diffuser from sinking is to run to your local craft store and buy up some sheets of plastic cross-stitch canvas. They are incredibly cheap and easy to cut with scissors. Cut to fit the inside of your egg box end to end. Put this under the light diffuser on top of the substrate like this...
This way you can make it very wet without sinkage.
I need to take another picture of one of my egg boxes since the only one I have fast access to that shows the plastic cross-stitch canvas also has a hatchling in an egg in it lol!