If there is a local glass shop that is good to work with, you can go to them and have them cut you a piece of plexiglass the size you need and have them sand the sharp edges for you. They have to be willing to be extra careful, and you need to check it before you use it. If all goes well it only costs a couple of dollars for that pc of plexi! I made my own diagonal lines on mine with permanent marker and wrote the square size on it for future reference.