I had this idea when I had my very first 3rd gen ages ago, and now I'm debating actually doing it. The idea is to make a false trunk, a la the Honda Ridegline in the location of the stock spare tire. You would cut out the floorboard in the rear cargo area, remove the stock crossmember (that the spare tire winch hangs from) and replace it with two smaller ones fore and aft, then weld in a "trunk" that hangs down below the frame rails a few inches, to roughly the height of the bottom of the rear bumper. Of course it would need to have a skid plate as well. Then you would cut a piece of masonite, thin ply or metal to make a false floor under the rear cargo liner. Would be a great (and secure) use of wasted space and the perfect place to carry recovery gear and stuff like extra fluids, tools, etc. Have also thought about putting a stainless water tank in that location.
Here is my old truck, by the way:
https://www.instagram.com/p/BYMjqIon...like_the_wheel