thats the bottom you're looking at - the cross members slide between the studs on the base of the odd compartment and the lock engages there as well i think
i'd go get mine and check, but it's in box in the loft right now and i can't be bothered with the hassle - getting a ladder, rooting around, getting dusty you know how it goes.....
Understandable. I'm not big on digging in attic space myself (tall, and I don't like being hunched over while trying to stay on the rafters to keep from falling through the drywall).
I don't consider that open, as it has the cross members on top (structural integrity and mounting points). But I didn't recall it appearing that open (amount of missing metal compared to essentially a solid sheet ).