Cut the Base
Use a plywood backboard as the base of the unit. This will be the piece the radius is cut into. The plunge router is set into one end of a swinging arm. The other end of the arm is attached to a board. That end is the pivot point for the swinging arm. Several holes are cut into the arm one inch apart, allowing the user to use the jig for several different radii. Cut a piece of 1 x 4 the same length as the base.
Two boards cut at an angle act as stops on either side of the arm. Attach them to the piece of 1 x 4. Use a band saw to cut the arm from 1/4" plywood. Make sure the head of the arm is the appropriate size for the base of the router.