One of the most useful jigs for cutting straight lines across panels with a circular saw is a piece of 1/8 inch masonite with a straight edge screwed to it on one side.
Make it a bit to big and cut off the excess by passing your own saw along the guide edge. Now the outer edge provides you a visual edge exactly where the saw blade will come through.
This cut edge is then lined up with your pencil marks on the target, and the saw can't miss.
Either screw it to the panel, or clamp the two ends.