The box was actually intended for 2 10" subwoofers. My issue was that either my subwoofers were slightly over-sized or the box was cut a bit too small.
I remedied that by filing within the hole to get it just the right size. In my opinion that was better than having it too large. As far as the space goes, the subwoofers are shallow mount subwoofers and Pioneer stated the ideal sealed enclosure size is 0.4 cu. ft. The recommended size is 0.35-0.7 cu.ft. This box is advertised as having a sealed volume of 0.55 cu.ft per subwoofer so it is good enough for the subwoofers.
One of my main issues issues is the crack in the box which I have sealed with some glue but it still bothers me because I know it is there and I will never be certain the the crack is still not affecting the sound.
Also to get better sounds from the subwoofers, I am going to unscrew the rear seat and put some washers under it to raise it up just a bit more.
In addition I will be putting some mounts on the bottom of the box to lift the base off the carpet and leave some space to allow the subwoofer room to push the air and create sound.
I will be working on that bit by bit when I get off work and I hope by saturday to have completed and test the sound quality.
I'll just have to keep my fingers crossed.
Eventually if it works as expected, I will purchase another box locally and do the same thing to remedy my doubts about the crack.
I definitely want to stick with the down firing design as I tend to agree that it will give me cleaner sound in a sealed enclosure.