James, they're not all that hard to make. Just make sure your DP is squared up first.
With the sub-base on and screwed on, make a photo copy of the bottom. Those phillips heads will help in centering the holes. Tape the photocopy to your plastic. I use a spade bit to 'center punch' the holes and then drill slightly over-sized thru holes for the mounting screws. My countersink is a large single flute cutter from Lee Valley. (single flutes make a smoother cut, IMHO). Mount the whole slab to the router and measure out from there to fit your recess. For the router bit hole, I chuck a V bit to make the 'center punch', remove the sub base and use a Forstner bit on the DP to make the hole. Hope that makes sense.
Joe`