If you don't ever really carry any large items (fridge, furniture, etc), you could always make an interior "trunk", hide everything behind the rear seats.
Make a horizontal panel on each side that is even with the trim, allowing a "cover" to rest between the two. The cover could be as wide as the wheelwells, which would allow you to still carry large items if you need to (just remove it). If you're handy, you could make a little cubby in each side, give you a place to put tow straps, tools, etc.
In my 4-door, I made enough room to put my subs on either side, made enclosures of about 2.4 cu ft each. I don't have a cover over my cargo area (as I carry tools all the time), but it would be easy to put one in place to hide everything from prying eyes.