How about for an 18x18x18 Exo Terra one?On my zoo meds I cut the glass to be slightly larger than the screen area. This allows for overlap so that the plastic supports the weight of the glass. It also allows me to slide the glass back and forth to control the amount of ventilation. I can control humidity and temperature this way fairly easily. This method doesn't require any modification of the existing top or an exact cut of the piece of glass.
FWIW - The screen area is 15.5" X 15.5". I cut my glass sections to 16" x 16" allowing for a 1/4 inch overlap. I generally keep the glass pushed back so that about 1/4 inch of mesh is exposed for ventilation.
Do you cut the glass smaller then?