My brother in law Trevor recently bought his first vehicle. It's an old beater Ford pickup, but he's pretty stoked with it. Arguably he could have bought more important things, but one of the first things he decided to get for it was a stereo system. He picked up a Sub, Amp and Head unit, and him and his friends stuffed the sub into a tiny box behind the seat.
When I first saw the box, my first thought was "holy shit, that's small," and I was right to an extent. It's a 12" sub, but it's actually designed to run in smaller enclosures (it works well in about the same size sealed enclosure as my 10" Kenwood)... but the box they had it in was just too small. I calculated it out, and it was only about a half a cubic foot, a hair under 50% of the size the sub is intended for.
It just didn't hit very hard at all for having a 300W digital bass amp feeding it. I theorized the box was choking it (among other problems with the setup, see below), despite the fact it was pretty leaky.
So we set about designing and building a decent (note: I don't consider the test enclosure I built for my Kenwood to be "decent") enclosure for it at the proper size, but trying to stuff it behind his seat still. We spent some time pouring over the specs, feeding it into WinISD, doing some Pythagoras (apologies to my Math teachers for my lack of attention) and we finally came up with a ~1cuft enclosure with a sloped face that fits behind his seat.
We grabbed some 3/4" MDF and started construction. I mocked it up with cardboard first, then started cutting. I'm not really excited about the quality of the cuts - all we have is a circular saw, as the tablesaw is dead. I took my time and re-did a couple and it has exactly one minor leak without any ahesive, baffling or sealant.
So yesterday we got it knocked together and test-fit it behind his cab. It sandwiches in great behind the seat, I made a spot to hang the amp (because it shouldn't be kicking around under the seat) and we tested it really quick with the wiring he's got. I need to pull it out and re-assemble it using glue, and then silicone the inside of it - I may do that today.
Then it needs bolting to the back of the cab, and the amp desperately needs a wiring kit. I've told him not to hammer it, because the wiring he's got powering it is probably 10-12 gauge (at a guess) and it really needs 8 gauge. The ground is also awful. It thumps pretty good already, he's not exactly looking for huge pounding bass - just wants to know it's there.