first off, welcome to the forum!
have you looked at something like the mtx thunderform or jl stealthbox? they both have applications designed to replace the factory enclosure and comform very well to the body structure and give good performance. if you stick a wedge in there you will most likely need to secure it to keep it from rattling or rolling around in the hatch. also you will prolly still have a fair amount of open space since it's a generic shape and not for that specific application.
2010 Volvo C70
2011 Harley FLHTK
Midwest Crew #22