Wipers! Not Ypres -- windshield wipers

Get a drafting compass (you can get pretty cheap elementary school ones at office supply stores).  Examples are here:

en.wikipedia.org/.../Compass_%28drafting%29

You can measure the length of the wiper blade with this, and use that to mark your ark on tape.  Cut the portion of your tape off the strip of tape you marked up and place it on your window.  Paint, splatter, or dull as you want and then peel the tape off to reveal your clear area.  

Gene Beaird,

Pearland, Texas

The distance from the "axle" of the wiper mount to the farthest tip of the wiper blade itself will be the radius of your outside circle. The distance from the wiper mount to the near/bottom end of the wiper blade will be the radius of your inside circle. Doubling each of these will give you the diameter of the corresponding circle, which is probably how your template is marked.

Cut out the inner circle and throw it away. The rest, cut into suitable sections, can be used to mask your windshield.

Cheers