I recently bought a 2008 mini cooper S and I have been having some mechanical troubles so i joined the community and I have a feeling there will be some great help here so I have decided to contribute to the community. I took some measurements and rean calculations in excel to give me total air volume of a subwoofer box fit as close to flush with the lower rear trim and not interfere with the rear seats. This is my initial design, it has 1.72 cubic feet of airspace inside a sealed box, the tool tray has been removed in order to get these specs. the most efficient way to add more space is to increase height, everything in the excel file has been formatted so that if any dimensions are changed the net volume of the box will change along with several other calculations but thats that most important number. I want to put a 12" subwoofer inside a ported box but for now I have designed a sealed box, this box would be perfect for a 300-400 watt 10" subwoofer, but it will fit a 12" and barely make the minimum air volume to rock nicely. I hope this will help someone looking to do the same in these little bitty cars!