X-ray film is usually exposed and then recycled at hospitals after it is used. Go to the dept and ask them to keep some for you, explaining what use you have for it. What I have received has been free. Since you are wanting something you can see through, try using a flat piece cut from the side of a milk jug. My mother used to save the plastic "tray" that is in most brands of bacon, wash them well to remove the grease.

Gonna sound crazy - but I use 12" vinyl floor tiles for templates - they are easy to cut with scissors or knives. They are heavy enough to hold the material flat and are easy to rotary cut around or trace. If any of the paper backing doesn't quite cover the glue area, just dust it with talcum powder and problem solved. You can cut out a section in the middle so you can see the material and place it exactly where you want it.

I use plain old cardboard and make myself a window the size I need. Tape the bandana down, place the cardboard frame over it where you want your square and mark it or cut it right there. Good luck!
Marian :-)

how many times will it be used? if you only need it for one project, use heavy cardboard and draw around it with a pencil or marker and hand cut. if you plan to use it a lot in the future, i think you should bite the bullet and get the real thing because you'll use it in other ways as well.

Most glass stores will cut a plexiglass template...

I have solved my problem. My local glass company cut me a piece of plexiglass, that way I could see through it. Was less then 8.00 for a 19" square, I didn't think that that was too bad.
Thank you everyone for your ideas.