I know nothing about subs and building sub boxes but I am in the same boat with the wires right behind my kick panel.

But Ian, given your diagram isn't the mounting of the sub going to be off center? I'm assuming that the top length and right side is hugging the interior wall of the hull? So the sub would be more at your right leg?

Are other boat manufacturers doing something different with the placement of that hole for wire runs? It seems foolish to put it right in the middle.