Building a pair of new Boxes for a small hatchback and want to try 4th order with a single 8" internal driver with 8" passive radiators used in place of the ports in each box as this will result in a better bandpass match with the satilites, and I have a few questions....

I did some research and I think I'd be better off with a 4th order bandpass.

I'm using a pair of Dayton Audio SD215-88 DVC drivers matched with a pair of Dayton Audio SD215-PR Passives, both have a Vas of 1.4 cu. Ft.

T/S parameters are:

Impedance : 8 ohms per Voice Coil
Re : 3.0 Ohms
Le : 1.19 mH
Fs : 33.7 Hz
Qms : 3.97
Qes : 0.51
Qts : 0.45
Mms : 32.03 g
Cms : 0.70 mm/N
Sd : 208.7 Sq. Cm.
Vd : 125.2 Lit
Bl : 6.47 Tm
Vas : 42.37 Lit
Xmax : 6mm
VC Dia. : 38mm
SPL (db@1W/1m): 86.4

Using AJDesigner, I substituted the PRs Sd for the dia. of the ports, and I got the following results:

Vf=1034.21 cu. in.
Vr=1714.82 cu. in.

These figures go well with the 4 ch speakers that will be at the 4 corners in the car. my best estimates give me a box that has internal dimensions of 10x22.5x8 inches for the rear sealed chamber with the driver in it, and 10x22.5x5 5/8 for the front chamber with the two Passive radiators.

I will be using acoustistuff to dampen internally.
I've included the vol. of the driver and passives in my calculations with a 10% increase is box size due to the damping material. The boxes will be internally braced in both chambers but how accurate do I need to be in building the box?

Since the box is over-sized, can I build something close and tune it with wood blocks in the rear chamber to get the right volume and mass on the Passive radiators to tune them?

Am I missing some other things that need to be included in the calculations?

I have a size restraint that says that both boxes have to fit in a space that is 12 in high and 48 wide, so I have to increase/decrease the depth to get a size I can work with. I have up to about 18 in to work with.
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Anybody Here?
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I used bassBox pro but it wouldn't let me use passive radiators in a bandpass. But I used 12" high 36" wide 18" deep. If you make the box 3 chambers the 2 sealed sides would be .788 and the ported would be 1.562. The port would be 4" x 15.75" which would tune the box to 35hz. Just keep the port 4" from the bottom of the box. The port will stick out of the box about 7". You could use a 3" x 8" port but you might get some port noise.
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