I made one that way. I just place a circle template on the glass and trace it with a diamond point.
The I just snapped off everything that outside the disc. Much to my amazement, it worked.
Maybe just beginners luck.
When I was bashing my Ruby Kit, I looked for headlight glass that was heat resistant. Someone here on MLS suggested mineral glass watch crystals. I found mineral glass watch crystals in many sizes from Otto Frei. http://www.ofrei.com/page773.html