Frank,
I don't have a design, but it is pretty easy to make. Simply trace your router sub-base onto a piece of thin plywood and include a "leg" or handle protruding from one side. Cut the center out to allow the router bit to pass through and replace your sub-base with the new guide. Measure from the edge of the router bit to a spot equal to the center of the circle you want to cut and then drive a small nail through as a pivot point. This makes a trammel guide for your router which you can set to whatever size circle you want by replacing the nail. Be careful with which way you turn the router around your wood. This will cut out a circle of wood. If what you want is to cut small circles out of a larger project, you can make a template out of mdf or plywood and use a pattern cutting bit in your router.
