$35 · Kreg Universal Rip Cut Circular Saw Jig

From the website: The Kreg Rip-Cut turns your circular saw into a precision edge-guided cutting tool that makes straight, accurate, repeatable cuts in plywood, MDF, and other large sheets. With the Rip-Cut, you can cut with the confidence you want and the precision you need. The Rip-Cut easily makes straight, accurate, repeatable cuts without measuring, marking, or having to keep the saw on the cut line. The Rip-Cut does this with a long edge guide that follows the straight edge of the sheet you are cutting. So, all you have to do is lock the saw in at the measurement you want up to 24 inch. using the built-in measuring scale, and then make your cut.