there are a few possibilties.
1) A box that goes under your parcel shelf. This can be difficult depending on the size of you woofer.
2) In the spare wheel well. Fine if you don't want to run a spare. IMHO, This could turn out badly... I am not a fan of waiting for roadside assist / family to turn up.
3) The fibreglass box that sits at the side of the boot. I went for this option until I can get the undershelf sub box the way I want it.
4) The premade sub box sitting in the middle of the boot. This takes up way too much space and make your boot pretyy much useless.
The box type is a matter of preference, be it either ported or sealed. I personally prefer sealed as it give a better and more accurate sound.