I use to do Stained Glass windows etc. That looks like tempered safty glass, and if that is a high powered light you do not want regular glass in it. The tempered safety glass is much harder to cut without real good tools. There are a few ways to cut a circular piece of glass out. Normally I would cut the circle with the line coming off of it, as has been said above. Then to finish it off to size I use a wet dimond stone, I have a glass grinder, but you could use a one on a dremmel or something but getting it wet does two things. One helps keep it cool, as the point of heat will often cause it to crack or break. The other is it helps provide a cushioning effect to the grinding . It does work. Not sure if it would work. But one of the water table tile cutter grinder may work also as they work on the same principle, but at a much higher speed.