Step 1: Gather Materials
A sheet of an unknown plastic. It was thin, very flexible and appears not to be very brittle. But more importantly, it was $1.75 for a 2' square sheet. I love surplus.
You will also need a cutting device - we used a jig saw with a fine metal blade.
A stiff backing - we used some scrap foam - this makes cutting easier
Step 2: Take Measurements - Mark Them
Marking your measurements is quite easy -- mark off the center of your circle by measuring your radius (plus an inch or so for safety) off of two sides of your plastic, then make a cross hair as shown.
Now, use a bit of string and your marker to generate a circle at the necessary radius. Just plant one end of the circle at the center of the cross hair and hold your marker at a radius' distance on the string. Then mark.