Like the other gentleman, I'm looking to add a little bass to my sub-less Fiero. At the moment, I'm eying a pair of 10" JL Audio TW5 Shallow Subwoofers. At only 2.5" thick, their shallow enough to slip behind the seats, unless the seat is 100% back, in which case there's not enough room. While the Subwoofers are small, the recommended enclosure size is .44 Cubic Feet or more. This presents a challenge. At the moment, I have a rough wood template which fits in the car wonderfully, with the seat only slightly forward (I'm 6'2", and with the seat where it is I fit in comfartably). The main problem, so far, is that the dimensions I have (18" tall, 18" wide and 4.5" deep, divided by two as it's a triangle results in a wonderful .42 cubic feet) fit the car, but not the subwoofer itself. With such a steep angle, the subwoofer needs an extra inch worth of thickness towards the shallow end of the enclosure. The way I see it, I have three options for solving this.

Make the enclosure taller, therein decreasing the angle from thick to shallow, to allow the Subwoofer in. However, this would become visible, which is one feature I was trying to avoid (I personally prefer an OEM look).

Option two, is to push the seat ever so slightly forward again, allowing me to extend the height lower a few inches, to the floor, as well as run a flat section purely for the subwoofer. However, I don't want to decrease the comfort at all, as it's one of my favorite features of the Fiero.

The last option is to cut out part of the firewall and weld in some aluminum myself. This allows the subwoofer to extend back, into the engine bay slightly. This maintains the stock look and comfort. The most worrying aspect, however, is that I need to expose the backing to the dangerously high temperatures from the engine, which would melt the fibreglass, and damage the sub as well. It's also alot more work, limits what can be done to the engine and makes wiring all but impossible.

At the moment I'm leaning towards making the enclosure only four inches thick, and then extending it down to the floor (another three inches). This nets essentially the same volume as before, and the seating position, however, I'm still not sure if the sub will fit in it. If you've got any good ideas, feel free to contribute. As a sidenote, once I work out the gremlins, I'll be happy to keep the templates in case anyone else here wants to upgrade their thump a little.

The left photo is the template, where I've crudely put in black lines to show where the subwoofer would approximately fit (it doesn't actually go to the edges, it just was easier to draw it that way) and the red areas showing how it goes beyond the sub box. The right is an unmolested version of the picture.

On a related note, I plan to correct the problem by slicing about half an inch off of the depth  deepest end, allowing it to slide completely past the seat, then I'll extend it another four inches downwards. With any luck, that will properly fit the dimensions, if not, I'll do some more splicing of the template.