@Patrick: I would love to here the Autotuba sometime! I dont know if you have any previous experience with the extremer
side of car-audio. Seeing that you are an authorized builder you probably know your way around things.
But when a lot of soundpressure is put on the car from the inside you need to take a few things in consideration:
1. Damping. And not just the sheetmetal in the trunk but especially the doors, roof and so on. Al surfaces that can vibrate, WILL vibrate
2. Foam-surrounds or wheaterstrips on the places where hard surfaces make contact. Doorpanels to doors, parcelshelf.
3. Create some sort of molding for any tools that are laying around (spare-tire space?) I did this buy putting a garbagebag in the spare-tire and then filling it with PUR. After you let the air out you can press the loose tools in it. It will grow when curing so beware of that. it does need some air while curing so don't leave the bag closed. after curing you can easily sand away the excess foam, lay some fiberglass and resin on the top surface, sand and paint and then maybe even install some straps, and your done.
I have done my share of SPL competitions and i know that anything above 132dB will shake a car apart in the long run.
My best install (in terms of sheer spl, nothing else) has won 2 trophy's 1x1st and 1x2nd place in the then called Unlimited
class. I scored 146 dB @ 54Hz. Setup consisted of 4 SPL Dynamics EXT-2000d Amps connected to 2 Atomic APX 12" DVC 1Ohm Woofers. Power came from 6 Hawker SBS60 batteries through 2x 50 mm2 powercables per amp.
Configuration was a half-wall. In a suzuki swift (geo metro i believe in the us).
That install did nowhere near 146 at normal music frequencies.
Those days are long gone, but i still love BASS. Ranging from House, Nederhop(Dutch Hiphop) to anything with a guitar to classical music and anything in between. Lets say 60 to 200 Bpm

On the other question: Yes i am into VW's. But i'm not one of those people who talk shit about other brands or dont wanna come near them. I just love everything that has an engine and i can find parts for just about anything!