To make this as simple as possible use the same size bit as the radius of the plate, if possible, if not use what you have to make a 1/4" BB template. Don't be overly concerned about the off set. Use the router plate ,with two faced tape,a plunging type bit and the guide of your choice to make the cut out in the plywood. Use the plywood template and the same bit and guide you used to make the template to make the cut out. Cut a very little deeper than thickness of router plate,see previous answer about using jig saw, use some means for leveling plate-set screws, woodhaven's levelers ,or something else.