I thought of this combo by browsing some enclosures made by Q-Logic/Q-forms and this forum for ideas for an arm rest.
Initially I wanted to custom make a subwoofer enclosure in the space behind the passenger's seat (trapeze shape), but I didn't like the idea of having the bass "locked up" in there. I think it would be greatly muffled.
I designed this "beauty" instead. Now all you engineers out there, I am not one of you. Nor do I have great computer or drawing abilities, but I think you will all get the idea.
I am also considering outsourcing the work to someone with more experience in woodwork than I have. I found a guy on ebay that builds all kinds of shapes for subwoofer boxes by request. (I'll post that link later, maybe there will be a group discount if more of us would be interested in this concept.
My design does not alter anything in the car and could be easily removed.
It would simply fit inside the wooden box between the seats with a possible location for an amplifier under either seat.
I designed it to be a downfire type of subwoofer, but it could also be made to fire toward the rear end of the car (but it would be visible, thus beating the purpose of having a hidden subwoofer).
There are plenty of options for a 6.5" subwoofer with prices varying from $30 to $300 (or more).
I found JL Audio 6W3v3-4 that I like as far as specs go and would fit this shape according to the sub's requirement for air volume inside the box.
My box would have 0.17 cuft of volume inside.
The interior volume could be enlarged a bit to fit other subwoofers with a larger air volume requirement by changing the height of the bottom piece of the box (lower for more volume and higher for less volume inside the box).
Of course it could also be lined with special subwoofer/speaker foam or other material to accommodate other requirements for different speakers in respect to volume (I know it does not increase the interior volume, but in stead it would slow down the sound wave, so the box acts as if it were larger...).
On top of it there could be some padding coverd by interior color matching vinyl or leather.