One thing i tried and it worked great is a big Sponge. The sponge spreads the water out and gives tons of surface area. The water evaporates quicker and it will keep your humidity maxed. I only use it when they are shedding.
If you calk the dry sponge to the upper inside of the cage it will be out of the way. And every couple days add a tiny bit of water to it.
I havn't had to use it lately because i added a small heated pond with a water fountain in my basement. Now my humidity is perfect in the entire room.