Ok I got it, then from what you say, and my needs for the trunk, its going to be one of two ways.
1/ the box sits up against the back seat with the sub and port facing the tail. In which case it will have the 12" clearance. But in this ill get the cancellation that the pictures illustrates.
Or
2/ have the box sit against the back seat with the sub and port facing the cab. This will give the port and sub 3-4" clearance instead of your recommended 12" clearance due to the 75 degree slope of the seat, but I believe from what you say should reduce the cancellation in comparison to 1.
So which of these two routes will give me the best two results?
Thanks for all your help.