In considering the design for an audio system in the STI, I’ve come across of a couple of pretty slick items. Space is obviously a concern, since having a trunk means learning to use the trunk. Fiberglass is obviously a nice choice considering the reduced weight compared to wood. Audio Integrations makes a couple of really slick items for the car:

A 12” Subwoofer Enclosure and an Amplifier Rack, both designed specifically for the car.

The pictures below are from the manufacturers site and are not my car. Pretty sweet items if you have a Subaru that they fit inside, light weight too.