Negative template from Positive template help

I am sorry if this has already been discussed but I could not find a thread. I will soon have a piece of 6mm toughened glass that I would like to embed into a kitchen bench top but I am not sure how to go about getting a precise fit, I was thinking of producing a negative template from the positive template (the glass) and then using a flush trim top bearing bit to cut out the bench. How do I go about making a negative template rom the positive template which would give me a precise fit. Is there a special bushing kit that would do this or another way of doing the job that someone could recommend. Thanks in advance Dave.