I don't understand how you are going to get a 8" speaker + a box into this space?!?!The slots are 6 inches from top to bottom. 7 inches wide and 7 inches deep.
That has precisely dick to do with why most cars peak around those freqs. If the speed of the cone movement had anything to do with it, they would peak at 20kHz in the audible spectrum. That's when a driver is moving the fastest. They peak there because that is where the transfer function of the car is the highest. It really has nothing to do with the sub and everyting to do with the car.The highest DB's come in the frequency range of 40-45 Hz because the cone is moving back and forth the fastest at those frequencies.
Most manufacturers provide a bass boost with a wide Q. It affects a broad range of frequencies. 45 Hz in in the center of the bass region. An octave lower and you are bordering on inaudible. An octave higher and you are in the midbass region. The bass boost at 45 hz falls right in the center and makes it useful for more people. Also most sealed systems start to roll off free-air around 45Hz. Most music stops having musical content around that freq and FM radio is only good to that freq. By boosting the lowest freqs present it makes it seem like there is more low bass when there isn't.That's why most manufacturers make bass boost that center between those frequencies