Yes the hardware shop is the place to go, or if you have a shop near you that replaces car windows, sometimes they have chunks of tempered glass that they'll bevel the edges on & you can get a good price on these. The glass is alot heavier than the plexiglass but if the price is right.....
Or if you don't wish to spend any money, take a sheet of newspaper & a carpenter T-square, draw yourself a square the correct size, cut out the center and use the "frame" section for a window. Lay your ruler carefully on the edge & either mark the fabric or cut with your rotary cutter.