I wondering the same thing the other day. Since I was already off my lazy a$$ looking at it, I looked from underneath. Right under there is the gas tank, drive line and muffler. (Driver side to passenger side.). None of which really extend high enough to justify the Hump all the way across the bottom of the rear seats. (Save the drive line hump found in most rear wheel drive vehicles.)
But one then must ask, what if that hump was not there? Well, the whole rear seat would be lower. There would still have to be a hump for the drive line. There would be very little if any space between the bottom of the floor and the gas tank, and you would probably have a heated rear passenger seat when you did not want one, right over the muffler.
So, unless there is something else between the driverside rear passenger hump and the gas tank, that is where I would try putting a small storage box. It will require great skill in cutting/hacking........Antman.......are you there?