I'm not familiar with exo tanks but if they have a screen top, put a damp bath towel or saran wrap over like 2/3 of it to restrict the airflow and hold in the humidity better. A larger water dish could help and add sphagnum Moss in there that you can mist down which will hold in the humidity as well. Hope that helps give you some ideas. Welcome to the forums, sounds like you're doing a great job so far. Humidity isn't something to stress out over too much unless your ball is having trouble shedding bc it's so low.