barryM
It can be done, especially if you are used to glass cutting, or have a lot of self confidence!
Wear eye protection and gloves - there will be fine glass bits and very sharp edges
You will CERTAINLY need to practice on some bits of unimportant glass
Basically you need to make a VERY sharp circular score line with a diamond - you will also need some straight lines on the outside of the circle.
Then you carefully but confidently break off all the outside parts!
Sketch attached of score lines suggested
To make the circular score I suggest a compass fitted with a diamond glass cutter (or better still a glass circle cutter). To fix the centre point a glass circle cutter uses a rubber sucker, but a resourceful modeller might use blutack and a pad of plywood or some such.
hope this helps a bit
andrew