Did you wet your substrate? I don't mean "did you spritz water on it from a spray bottle?" I mean actually WET it. Pour a whole pitcher of water over it and mix it up with your hand. If your tort table isn't water proof or won't hold water, this is going to be messy. You should probably line it with plastic sheeting, then pour water over it.
The material you used to make the top part would likely work as a lining for the inside of your table if you lay that across the bottom and up the sides. Then just fill with substrate 2 or 3 inches deep should be fine. The more substrate the more moisture can be held and released.