The JL CS110R-W0 PowerWedge system is a good sized subwoofer to compliment the audio system. It is a 10" subwoofer in a sealed enclosure for tight bass, as opposed to boomy bass systems heard from passing SUVs. I chose the enclosure intentionally narrow, but wide to maximize trunk space, through using the seat passthrough. This subwoofer may be easily removed if carying substantial amounts in trunk, or of course easily permanently removed.
You can see that the way the sub fits snugly, it works quite well in the trunk.