I saw a write up showing how they cut out a circular plate out of the rear trunk shelf, as an alternative to leaving the trunk access open for better sound quality for a subwoofer.
If you look inside the trunk at the ceiling, there is a 8-10" diameter black metal plate (4th gen legacy, not sure of other years). The idea is to cut that out, opening a port to the inside of the car, and cover that hole with speaker mesh to keep it looking clean.
I made the mistake of not bookmarking the page, and now I can not find it. Any of you have it saved by chance?