Fold a large piece of paper in quarters, use a plate or a compass to round the corner and cut out.
I have one of those cheap rolling rulers that can act as a large circle tracer. I put my fabric or paper on a surface that is firm but I can put a pin into. I put the pin in the hole at one end and put pressure on it. I choose another hole down the side of the ruler for my pencil point and trace around.
Do you have a pizza pan large enough to trace? You can also make a cheap compass by putting a pencil on a string and holding the other end of the string at the center point and tracing the circle edge with the pencil.