The bass section from the Klipsch Cornwall uses a modified motorboard design that moves the ports from the bottom to the side. This allows the 15" woofer to be mounted directly in the center of the motorboard. The assembly is a two piece design. If I build another pair I think I will change a couple of things. One, I will mill a rabbet for the outside diameter. The 3/4" drop is slightly too deep, and the 15.625 diameter is overly large. Other than that the assembly fit the speaker perfectly, and a test fitting to the cabinets was also perfect.
Here is the double sided tape method I use to cut the circles out of the MDF. This attaches the MDF to the backing/waste board so that when the circle is finally cut free it doesn't move around and cause a problem.