ironchefmitty Dear Ironchef!
Please, never cut an oval when it's on the canvas or you have a great risk that you will cut into the canvas!
The backside of contact paper consists of paper where you can draw (pencil, marker, pen ...) the shape on that you like/desire. There are several ways to draw an oval, two of them are explained in an article.
It's also possible to put an oval shaped object (bowl, canvasboard, ....) on your contact paper, draw around it with a pencil and then cut it out.
Best of way of cutting, at least what I do, is to cut aboaut an inch out of it with a cutter (follow along the line of course) and then stick some scissors in it after which you can start cutting the entire shape out. The slower you do this, the more accurate your shape will be.