I picked up a couple of cheap 8 inch subs to help fill out the bottom end in my RX-8. Not sure yet what the install will be, hoping they will fit between the back seats in an IB configuration.
When I picked up the speakers I had intended to install both in this location, because I thought I remembered seeing an install like that. While the space is physically large enough to fit both drivers there (particularly if mounted basket-out), the layout of the plastic would make it a pretty tight fit. It would be possible to strip it out that stuff for a fully custom install, but I'm not going to get that deep into it.