For keeping humidity in you can get a glass cover for the top of the tank, cut plexi glass, or glad press n seal or saran wrap are the most effective ways of keeping in the proper humidity. About 90% of the top should be covered to keep the proper amount of humidity in which is about 75-85%RH.
I'm confused, you guys say you saw your crabs molt? or exoskeleton?
They are suppose to be able to bury in the sub for this process in their life. They molt many times during their lives. It's suppose to be done under ground given proper conditions.
I currently keep crabs I currently have four species. Strawberries, Violas, Ecuadorians, and Purple Pinchers.