A cheap solution is a drywall square and a couple of c-clamps to hold it in place. Just use the square to mark a square edge, move it over the offset of your circular saw and clamp it in place. Use the clamped square as a guide for the saw. Works as a router guide as well.
If you have a lot of panels to do, one of the solutions mentioned by others is probably the way to go, but for only a few... |