i designed the box to the manufacturer's specs to within 250ml (.25litres) of their recommended size & my dad made it up for me. so i think we did a good job, as its within the size of less than a coke can of their specs. it was a difficult process though, as the required size for my sub using a sealed enclosure is only 27 litres.
the box is angled to match the incline of the back of the seat. its easy enough to pull the seat down. and it only hits the back of the seat when the stereo is excessivly loud, which doesnt happen often. its not a big problem.
what i meant with the whole muffling/muddy sound sometimes occuring, is when the cone is on a positive excursion (moving toward the seat) it pushed air... this air hits the seat, and has to go somewhere, while the sub has made its negative excursion (away from the seat) and is about to begin its next positive excursion, some of the air that originally was belted up against the back of the seat sometimes bounces back off the seat & towards the sub cone again, therefore, while the sub is making a positive excursion, some of the air wants to force it on a negative excursion, therefore, making the note inclomplete/distorted. does that make sense?