Coach, if yu are talking about custom building a fiberglass enclosure and thickening the crap out of it, sure, it will work. BUT, the way fiberglass resonates, it will have to be HELLA thick. For higher toned speakers, it might be ok, but I am more than skeptical with a sub enclosure. There are ways around that though. If your enclosure is just for looks, spraying some gunite or solid poxy-fill on the INside walls will solidify the shell. VERY expensive though.

And you arent kidding about the concrete being slightly overkill. Years ago, we had a job doing a custom enclosure for, of all cars, a Caprice wagon. The most hideous lowrider I had ever seen. The rear suspension was juiced up just to handle the weight of this enclosure. It was about 300lbs of concrete molded in a wood and fiberglass enclosure for form fit. It came out to 3 pieces and then secured in the rear of the car. 2 15" subs were mounted in them with cheesy green neon lighting and carpeted with white. It sounded freaking incredible, but wow.... the price you pay to have a grocery getter sound good.