Go to your local glass company and ask them to cut you a plexiglass square the size you want. Tell them it has to be exact because of what you are going to do with it.
Or, use the 12 inch square you probably already have. Fiddle with it long enough and you'll see that it has nine inch lines all the way around and you can lay it on your fabric to get any size you want that doesn't exceed the size of the ruler. froggyintexas