It might be a bit of a project if you're not big on fiberglassing, but I did an enclosure for a buddy's Acura TL with similar goals. Built a fiberglass enclosure for a 10" Alpine Type R in the drivers side (although you can go either side of trunk, doesn't really matter) of the trunk behind the wheel well. Still allows for 95% of your trunk space to be kept, and gives you a good sound and a custom look. Just make sure when you're building it to leave yourself enough room to get to your tail light bulbs without removing the whole thing.