um do a ported sub box with a 6" pvc "aero port " going through the center of the rear deck , flush mounted Finish it off with fiber glassing the rear deck carpet to contour the ports flare .
i reccoment against this but its your car . you could also try sub facing up port facing forward with your seat down . it acts like a poor 6th order bandpass box .
or just eliminate your trunk like i did ... not that big of a sacrifice