I think the box you are referring to is the cartridge that holds the ends (and aligns the rollers) of each pleat in the early model acordian style cargo cover. It looks from the photos that you have the one on the right end, but not the left. The two boxes, or cartridges, are part of the cargo cover itself. Once the cargo cover is unclipped, ie when you are loading large items, those cartridges can fall off. If you have both ends, they can be reinstalled on the rollers. I had to do that a few times. If they have been disassembled, probably by someone trying to get the rollers back in, then you just need to ensure that you have all the parts. If not, buy a new cargo cover.

Edit: Looks like you can buy just the ends. They are called supports, and are different for left and right, available in three colours. Item 14 on the drawing in this link:

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