I do not understand.I can see that you have prevented the back wave from going into the cabin through the back seat but what prevents it from going up and through the rear deck and also what prevents it from distorting the rear speakers?
Flat to 20 HZ - you could sign me up for one right now
I just do see how it would really work that well.
Back in the day I heard plenty of IB setups, and not one of them sounded good at all.
Granted none of them were professionally done - nor were the subs near the quality of todays subs.
I think your install looks great and you have great skills - I am just not an IB believer (yet I hope)