I have heard of putting moist soil in the bottom of the broody nest box also, from a book called Hatching and rearing your birds, it is very helpful. It says that soil helps humidity levels or you can also sprinkle some drops of water on the eggs a few times to help. It also tells a story about a hen they used that hatched out them out fine but then the hen turned on the ducklings and began eating their lil webbed feet! By time they found out some ducklings had died, others had to be humanly put down, with only a couple still alive and well. So if you do use a hen be sure to watch them closely to make sure the hen is really a good mom and wont go bad on her lil duckling brood.