Print 3 of these and use M3x12 SHCS and nuts to attach them to the frame. You can use 6 if you'd like, but 3 gives better results, because it doesn't over constrain things and subtly bend the bed. This works with 5mm thick pieces of glass, and I use the 300mm diameter ones from RobotDigg: http://www.robotdigg.com/product/490/300mm-Borosilicate-Glass-for-3D-Printing Here is a link to the KosselPlus printer design: http://www.kosselplus.com/