cut a piece of hardboard or paneling (good, thin plywood, plexi/whatever can be used, to) 6" wider than the base of your saw. glue/screw (countersink) a 1x4 to it from the panel side offset enough that you r saw can trim an inch or so on both sides. you can use a 2x4, too, but i like the sharp square sides, instead of rounded. set your depth so it just scores the surface, like 1/16" to 1/32" deep and make a pass on each side of the 1x4, using it as the fence to ride the saw base on, going the same direction. set the blade full down and finish trimming the panel. take your time for a clean cut, and you now have the perfect clamp-on guide for your saw, with a clean reference edge.