can you post pics of the tank exactly the way it was when it cracked? also, a pic of the 3 other side from the outside? drilling won't work if it is cracked. it will most likely shatter if you try. it is feasible to remove just the top and replace it with some sort of screen, but it is FAR from easy. how are you misting the tank?
btw, heather is correct, even though it is a uvb light, it will still give off heat. it does not take much of a quick temperature swing to crack non-tempered glass. all you need is a small hotspot and some cool water and voila! instant crack. the pictures should help solve what happened though.