I went to Home Depot and bought a 6 pack of 12'x12' mirrors for $12.00 and a glass cutting wheel. Then, like dacb; I scored and snapped them down to 10"x10" and trimmed the corners by scoring and snapping with pliers. Then sanded down the snapped edges with 220 sandpaper.

I noticed with the mirrors that my actual bed temperature (measured with Fluke laser thermometer) is 10 degrees C less than what I ask for. Set for 110C but measure 100C. Not sure if this is the same for glass but thought I should note it.