It's been a bit of trial and error with what we use in the nesting boxes. We have had a ton of rain here in Perth, and the boxes while well sealed still get damp from time to time. I discovered this "confetti" the other day, which is ideally aimed at guinea pigs etc, but thought I would give it a go for the chooks. It says it absorbs 40% moisture plus odours. The chickens weren't too sure what to make of it at first, but soon accepted it.
Omelette decided she didn't like to be in on her own, so got out and hopped in with Phoebe!