Please don't mount just glass. The stuff to use is called pyroceram. It is fireplace galss and is MUCH safer than other glass. You can buy it from knife maker supply companies, but check your local glass shop. It will tell you something about the business. They have no other use for a piece that size. If you are in a hurry and want the piece cut to your specs, expect to pay like lobster - but if you can be patient, a good shop will be happy to give you a good deal on their 'scraps'.
Rob!