waste of time IMO.
you can get much better sound quality from just one 12" decent sub in a box in the trunk, than from a free-air sub in the back deck. you have to brace everything after cutting, you'll loose rigidity, and flat out, you will not get sound quality worth all of the hassle involed.
be different, go ahead and do it, but if you want to do it for sounds, you won't be happy.