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To make one, cut a guide from a straight piece of 1/4" or ½" plywood that’s 3" wide and 4' or 8' long. Then cut a base from a piece of ¼" or ½" ply that matches the length of the guide and as wide as the width of your circ saw’s base plus 3" plus the half the width of your router base. Glue the guide to the base so it’s parallel to the long edges of the base and located from one edge a distance equal to the width of the circ saw. Now, equip your router with a straight or spiral bit and trim the router side of the base as shown. 

Trim the circ-saw side of the base in the same fashion by guiding the motor side of the base against the 3"-wide plywood guide. With your straightedge completed simply place its cut edges on your mark to saw or rout where needed as shown. If you switch to a smaller bit you will need to use the bit, rather than the routed edge of the straightedge, to eyeball the position of your cut. For quick reference later, mark “router” on one side of the straightedge and “circ saw” on the other side.