There have been a number of other threads on this topic. A custom box from a local dealer will be over $1K. el_duderino is considering making an off-the-shelf box but it is not clear if/when it will be available. I'm not looking for earth shattering bass just want to fill out the bottom end. Do you know what the maximum depth is? JL audio has a 6" sub that might be a good replacement for what comes in the box...

http://mobile.jlaudio.com/products_s...p?series_id=23

Nominal Diameter Overall 6.5 in. / 165 mm
Diameter (A) Mounting Hole 6.86 in. / 174 mm
Diameter (B) Bolt Hole 5.625 in. / 143 mm
Circle (C) Magnet 6.125 in. / 156 mm
Diameter (D) Mounting 4.54 in. / 115 mm
Depth (E) 3.5 in. / 89 mm

Dimension (b) above is what the holes in the box needs to be. Coupled with a high quality amp this could produce some nice tight bass..