A rafter square saw guide will have you raising the roof

Make square crosscuts and flawless 45° miters effortlessly with a circular saw by building this adjustable guide. With the circular saw tight to the square, loosen the wing nut to slide the scrapwood fence over until it overhangs the saw's blade. Tighten the wing nut and trim the fence to exact length. You can then align the fence's edge with the workpiece's cut mark for a precise starting point.
—John Stahr, Chicago