I've seen this done before with a 10" sub but I thought I'd try it with a 12" I Had lying around.
This is basically how it went:
Removed the 'indent panel' on the RHS in the boot.
Cut out the panel with a sharp stanley knife
Cut some MDF to size
Fitted the sub and a cage cover;
Painted it (brown is the undercoat, I went black after)
Put some of the latex stuff on the plastic panel to make sure it won't rattle.;
Put heaps of cotton wool and expanding foam inside the panel to seal it up. Then got some latex door seal rubber to put around the MDF and fitted it in (very tight)
Result (sounds awesome!), still needs some carpet or something to make it look neater, the painted MDF doesn't cut it:
Total cost: $13 for paint and latex door seals.
MDF: side of the road
Sub: My shed
Wires: as above
Harry