^ I also remember articles on a couple of 911s with that same woofer mounting - under/below the back seat with the woofer firing forwards.

I've never been through a 911 myself though so I can't say for sure.

 

did 2 different porsches, both 911's both with avi 10" woofer's in the floor, the ss500, only 2" deep, the box was in front of the passengers feet, lost a bit of legroom, but not to substatial

 

There was a company called I beleive Z-Box that made a subbox that fit in front of the bottom rear seat cushion, not sure if you could find a sub nowadays shallow enough to mount in there though, and also not sure if they fit the newer models, the ones I saw were for older Porsches.

The only Porsches I have done (all 2 of them, that's all there is in the Hat) were just basic deck and two's on older cars, but I think a 10 in a small sealed fiberglass box in the passengers footwell could be an option depending on how tall the guys mistress is.