I looked into it, but realized that with the thickness necessary for MDF, combined with the lack of space, you dont get that much extra volume using that space.
That and I wanted the extra floor space on teh passenger side when the seat was raised up.
So, I made one using a 10" sub below the drive side seat. Looks great, sounds great, and was cheap to make.
Here is the DIY that I wrote:
How To: Custom KC 10" Subwoofer Box ( '04 Nissan Titan King Cab )