Why dont you just get yourself a sheet of 300mm x 300mm x3mm borosilicate glass, and then buy a glass cutter and cut of the 80mm each side to fit?

Glass cutting is super easy, just get a very straight edge (preferably a square) and then gently run the glass cutter over the glass 5-10 passes, this scores the glass nicely. Then all you need to do it lift the glass no more than 1/2" up with the large straight edge beneath the score now, and gently push "evenly" down on the 'cut-off' side and it will cleanly snap off.