Fast to set up and highly accurate, this cutoff jig was designed for 6" boards. But you can increase the width for larger workpieces, or angle the fence and enlarge the jig to make 45° cuts.

To make one, measure the distance from the edge of the widest portion of your circular-saw base to the blade, add 14 ", and mount the fence that dimension from one end of the base. Then, with your circular-saw base against the fence, cut the jig end to final length.

To use the jig, clamp it with the stop against the workpiece edge and the right end of the jig aligned with your cutline. Hold the saw-base edge against the fence as you cut, as shown above.
—from the WOOD shop