If you don't have substrate, you'll have to find a way to catch any excess water. Substrate isn't terrible if you top it off with a decent layer of oak leaves and can go a long way to soak up any excess water. It will have to be swapped out fairly regularly though, since you shouldn't have soaking wet substrate either. It's a balancing act of how much water goes in and how much you can remove in that time and having a glass enclosure makes it difficult if you can't drill any drainage holes.
Luis