bob, most people I know just use quarter inch masonite or plywood for this. Cut a piece of wood as wide as your base, a little over half the diameter of the biggest circle you want to cut and taper it from one end to the other. Transfer your router hole pattern to the wood and chamfer for flat head screws then drill the center hole. You can drill a screw hole at half the diameter and insert a short screw thru the circle guide and slightly into the workpiece. This works and is cheap. I don't know of a commercial unit out there like you describe. Sounds like it works great but I don't know how many they could sell as cheap as the other method is since you can usually find a piece of scrap long enough.