I wrote an article on my site about how to make an incubator like that.

http://www.glassreptiles.com/articles/incubator.asp

If you do it correctly, there will be no need to worry about humidity. The entire thing is 100% humidity. Get your temps right and set that thing up a week in advance to allow it to stabilize. Once a week, air it out for a couple minutes. Good luck!