Common problem with many printers that use the Prusa style heat bed with mounting holes. Options I know of are to buy glass with the corners rounded (eBay etc), buy window glass and cut the corners diagonal to clear the hardware, or use a rectangular piece of glass that sits between the rows of screw hardware.

On my MakerFarm i3v, I use 3mm window glass cut to the size of the heat bed. Corners were cut diagonally to clear the bed hardware, and I use mini-sized binder clips to attach the glass to the bed near the diagonal cuts.