You could cut a piece of template plastic into that size and use it for a pattern.
I put sandpaper dots on it to help prevent sliding on the fabric.
Because I have been known to nick the thin plastic, I put an acrylic ruler on top of it and rotary cut against that.
You could also go to a hardware store that does window repairs and have someone cut a piece of plexiglas to the size you want. Just check the size before you leave to make sur it is very close to what you need. Some people are better cutters than others. Stress the importance of accuracy to your project.
I would mark either of these templates with the size so I would grab the right one.