Just another idea...
This might not be large enough for you but I built two locking, rain proof boxes between the wheel wells back in '00. They are HD as the Vista fold and tumble seat bolts to it at stock height and the seat belts also attach to them. I usually strap two ice boxes behind the seat one for food/snacks overnight (hopefully not needed provisions) and the other holds clothing and keeps them dust and rain free. I've gotten older (actually, my wife loves the comfort of heat and less wind) so I don't run the bikini top year round anymore-12 yrs straight till I built a custom 1/2 cab with windscreen canvas instead of the normal steel back.
They are wheel well to wheel well width, and 3/4 of each top hinges upward (worked perfect for my needs). I didn't want to lose the space with rollers and have to deal with stuff falling behind or off the sliding shelf. The front box holds spare parts that I seldom if ever need-axles, drivelines, ujoints, hub tools, ARB line, electrical, etc. The rear box extends to the tailgate and holds recovery stuff, plus small tools. I keyed them both alike with deadbolts the same as my house. POR'd them back then and they work great. I had to narrow it to fit the new wider wheel wells now with the new suspension/ORI build.
My Golden Retriever and my wife's dog get the area behind the front seats to the cooler area.