I'd had the idea of "probably best not to screw through a brake line", but leaving it loose raises the question "what do I fix my racking to"? If I fix it to the ply floor, it would only be as stable as the loose floor.
I currently don't store anything overnight in the van as it hasn't got a solid bulkhead - all to easy to see through for valuables. I was thinking of making a ply bulkhead too, which would need fixing somewhere. I might be able to fix at the roof and sides - it's not to take any weight other than the ply, and only to act as a partition (if I paint it black it'll probably look like a metal bulkhead from outside which may deter people with an eye for putting windows out)