Battery Relocation, and a pair of 8" subz will fit in neatly under the parcel shelf, however you have to consider the Hicas unit on the top right of the area.
But then you will need to relocated the battery, somewhere in the boot most likely. This occupying the same amount of space or similar to a sealed 12" box floating in your boot.
You just have to weigh up things, work out how far you want to go with the whole install.
Also a good thing to do before putting an enclosure under the parcel-shelf, would be to upgrade the Fuel Pump. Saves messing around later.
If you want drop me a pm, and come past my garage, and i can show you a few things?