My friend (thanks Josh!) and I came up with a design in Bass Box Pro for a subwoofer enclosure for my 330ci:
The box will be mounted under the parcel deck in the top of the trunk where the OEM subwoofer enclosure is normally installed. The box is designed to work for the music I listen to (Punk, Rock, 311, and Sublime) so it is tuned to be more musical and tighter on the bass response. The port is 58″ long (measured on the centerline) and the box is designed to work for an 10″ Alpine SWS-1023D subwoofer. It will be run at 4 ohms, and pull 200 Watts. It should sound perfect for my musical tastes.
The amplifier will be mounted in the spare tire pocket in a two piece assembly. The bottom piece will provide the base for the amp to attach. The amp will bolt to the top of the bottom piece and the crossovers will be assembled on the bottom of the bottom piece. The area towards the front of the car will hold my road side tire repair kit (mini-compressor and tire patch kit).
The top piece will slide over the bottom section and cover the wires and support sections. The only visible sections will be the top of the amplifier and the tools.
I plan to flock all of the assemblies in charcoal grey. At the very least, I will build the amp mount bottom section this weekend as I really want to use my amp. I think the top section will take a bit more time to do as I will need a rather large piece of 1/2″ MDF. All the images are clickable to the larger E-size prints.