I'd do a good old B-pillar flat wall, that way you have back to the cargo area for the enclosure, and you've got room back behind for batts/amps/etc. 4th doesn't leave much room for that stuff.
2004 Chevy Aveo-
Alpine-Eclipse-Vifa-Polk-CT Sounds-Sundown-Knu Konceptz
Essque on a budget
Team Quiet/Team SPL Farms