If it has a plastic top it should be able to maintain humidity about as well as a glass aquarium. Some people use wire mesh cages. These cages give the crabs the opportunity to climb, but they don't retain humidity, so I don't recommend them. Be careful because a hermit crab would be willing to leave its shell in order to escape. The bottom of the cage should be covered in about an inch of either sand or gravel.
Sand is also easier to clean.
It might be a good idea to have an extra, smaller cage to use for separating crabs when one is molting. Your pet's home should include choya wood or coral for exercise. The cage should not be placed in direct sunlight or the crabs will be baked. If it is near a heater the crabs may slowly suffocate because of lack of moisture. I have also created a checklist that outlines the basics of hermit crab housing.