Cereal boxes have great square corners to start with. Measure, measure, measure, then cut! Most older quilts were made with cardboard templates!
Picture frames that have plexiglass are good sources. Another source it to go to your local hardware store and ask for a scrap of plexiglass. If you are really nice, the guy may even cut it square for you! They have good measuring tools! Just decide on the size you want and ask! The worst they can say is "No!"