Circle Cutting Jig for Circular Saw

Cut a piece of 1/4" plywood about 12in wide and as long as your largest needed radius.

Attach the 1x2 guide rail to one edge of the base with glue and screws, leaving about a 1-1/2in overhang past the rail.

Trim the overhang flush using your circular saw's own base plate riding against the 1x2 as a guide — this calibrates the jig to your specific saw.

Find the plywood's width-wise center, measure out your desired radius along its length, and drill a pivot hole sized to account for your saw's blade offset from the base plate edge.

Pin the pivot hole at the center of your workpiece and swing the saw around it to cut the circle, clamping the jig down if it tends to rotate.