I would suggest putting matting down and ensuring it was completely sealed and then it might sound decent. Bass tubes would be a better way to go if you are thinking space (as your other post suggests) Put one on either side and you can put all your shopping/gear in between. Just make sure they are secured in place before hitting the track.
You can put a sub in the spare wheel well.
Your best option would be to find a suitable sub first. You'll need to calculate the area of the wheel well and also decide whether you want it sealed or ported.
Dynamat is the stuff you need to line the wheel well. It will sound a whole lot better with dynamat inplace.
sub 
Linear Mode
