Quincy
You asked me to explain how the plinth is dimensioned, and assembled. I refer you to page009 of the instructions:
(1) The sub-plinth (P1) should measure 196mm wide and 304mm deep, which is exactly the same width and depth as the chassis. It fits directly to the bottom of the chassis.
(2) The sub-plinth has a cut-out 156mm wide and 264mm deep to provide access to the XO board. There is provision at each corner of the sub-plinth for threaded inserts to be installed, so the main plinth and be bolted and unbolted at will. This is how you get access to the XO if you need to fine tune the tweeter response.
(3) The sub-plinth is glued directly to the chassis, and should be flush with it if you are dimensionally exact. I suggest you glue P1 into place only after you have shaped P2 and aligned the bolt holes. The other tip I can suggest is to over-size P2 so you can trim it down to the exact cladded dimension of the finished loudspeaker. The diagram on the bottom right shows P1 stacked on P2.
(4) Your ABW cladding overhangs the sub-plinth to create a smaller gap to the plinth. I think I made my gap 10mm. Paint the edges of P1 before installation.
I hope that clarifies your question.
James