Because the box my sub came in was too large really (needed to be able to fit a buggy in the boot) I just custom-built a smaller one, which is shaped like a pyramid with the top chopped off, and the sub sits in the bottom pointing downwards...
I just made some little round 'feet' out of MDF for the bottom, and put screws through the centre of these before screwing them to the box, so it has 1/4" spikes sticking out the bottom. The feet ensure the sub isn't actually touching the carpet, and the spikes stop it from sliding around in the boot. The design (sub in the bottom, and narrowing towards the top) keeps the centre of gravity easily low enough to stop the whole thing falling over.
I stuffed it full of Cotton wool as that allegedly makes it sound like a larger box (and it does seem to work, sounds better than it did empty)
Some people get really stressed out about making sure the box is exactly the right shape and size... but I find this box doesn't sound a whole lot different to the box the sub came in. The important thing is that the sub is boxed (unboxed it sounds cack) The footprint of this little box is only around 14"x14", and that's with a 12" sub, and with the Amp screwed to the top it takes up hardly any space in the boot, plus the 'pyramid' sides mean it sits up against the back seats nicely, and I can easily remove it and stick it in another car without leaving any unsightly holes anywhere.
It looks a bit gash, but thats just cos I can't be arsed to carpet it at all

king of the bodge, me.
