A plastic model of the Aluminium storage box is a good idea..
In our model of camper we can access this area on both sides from
inside the camper via two doors on hinges...
We have 4 duffle type insulated bags that we wrap in heavy plastic bags in those two areas of the truck. We use insulated bags for shock absorption as sharper objects tend to mark up the camper tub from the motion.
If we had a plastic version of the storage box I would have a hole cut in the side as well as the rear access door, with our present system it is hard to reach that area of the truck box from the back of the truck , the box would solve that problem, the plastic bags get a bit dirty from dust after a while..
The area opens up a relatively large storge capacity for us...
We stuff it full of things that are not in every day use.. Maybe too much "stuff" as we gross about 11,500 going down the road...