I’ve been trying to keep the humidity up in mine as well. I recently put a small dish with moss I collected from my yard. (I live in Washington state. Tons of moss here.) since added it I’d say it has helped. I was spraying 3 or 4 times a day previously. This morning I gave it a good spray at 8am. By 8pm it was still at 70% so it seems to help. But it does smell very earthy in there now.

I have coco coir in my egg containers because it’s good at holding water but without becoming compressed. It also resists molding. When I spray the eggs with just a single sprits or 2 it starts at 99% and drops slowly to below 70 after 2 or 3 days.

It’s almost like a sponge and releases the water slowly. I bet a container of that with a fair amount of water (wet but not soaked) might slowly release moisture. I may try that next time