My TJ had the rear seat removed, and the previous owner had built a wooden box that was just narrow enough to squeeze between the wheelwells. Took up the whole rear of the Jeep, from behind the front seats all the way back to within inches of the tailgate. Was about a foot tall. Lid was hinged with a prop rod, and he had it opening to the front, but I flipped the box 180 so it would open to the rear.
Had everything in there except my toolbox and one towstrap and my hi-lift. Spare axles, spare fluids, extra straps, spare unit bearing hub, shovel, bottle jack, some bigger tools, etc.
I could strap my toolbox down on top of it. He carpeted the top so his dog could lay on it when he wheeled. I was thinking of raising it up off the floor so that my 9.5-inch wide 33-inch spare would fit under it and I could get it off the tailgate. But I sold the Jeep before I did that.
I don't think I have any pics of it, but if I find some, I'll post them.