Is there a recommended depth from the bottom of the glass top to the top of the sand in a ZenXY build?
Between the bottom of the glass and the top of the surface the sand sits on, you need room for the ball and the sand. I like to use baking soda and keep it under about 1/3 the diameter of the ball. The ball will push through the sand. So probably 1.25x ball diameter would be a decent minimum.
More likey though, if you add LEDs, you will want a little more, like 2x so there is room for a little bit of an angle above the sand. I would also guess a larger table would benefit from a bit more gap.