A viable design, but not for a car!
It's funny but not because the design is "wrong" since that's actually quite close to the result that I got from an enclosure design program (the 4 ports need to be around 30 inches long though so they do take up some of the cabinet volume). However, this is for flat response with F3 at 24 Hz in an anheonic (SP?) chamber, with the average "cabin gain" you get from the small interior of a car there will be a major hump in the lower end of the frequency response. Though ported enclosure designs are more efficient than their sealed counterparts, below port tuning the driver is "unloaded" and can easily go beyond excurtion limits, not to mention having much reduced power handling. A sealed enclosure subwoofer with an F3 of around 40-50Hz will usually have flat response down to 20Hz from the cabin gain, and though not as efficient as a ported design, will be able to handle much more power at all frequencies. Another thing to consider is the sound quality, ported speakers tend to sound boomy because the enclosure provides very little damping to control the driver which can result in overhang at certain frequencies especially for drivers with heavy cones.
In short, it's funny because he wasted all his trunkspace on a design that's not optimized for his car =)