If you are worried about the rock den getting too hot, put it on the cool side of the tank away from the heat source. Your humidity hide should be placed on the warm side of the tank. If cost is a problem, then you can use a terra cotta plant pot (turned upside down). If you want something a little fancier, there are some great caves and other accessories at
www.organicfantasies.com I have several pieces from Margie and they are all top quality. She will work with you to create what you want (color, shape, size, etc.)