a 15! HA thats kiddy stuff. What you need to do is replace your hatch window and mount it directly on the hatch. Your Z will be the enclosure.
Put this in your Z
specs:
Description 24" Square Subwoofer
RMS Power (Watts) 4,000
Rec. RMS Amplifier Power (Watts) 2,000 - 4,000
Impedance Dual 2Ω
Frequency Response 18Hz - 150Hz
Voice Coil 6.5" (16.51 cm)
Magnet Weight 56.25 lbs (900 oz)
Mounting Depth 24" (60.99 cm)
Cut Out Diameter 22 1/4" (56.52 cm)
Volume Displacement (Sd x Xmax) 653.29 cu in
Speaker Displacement 2.06 cu ft
Sealed Enclosure Net Volume 5.470 cu ft
Vented Enclosure Net Volume 10.2 cu ft
Tuning Frequency 29Hz