Ply Lined Van - Screw Length?

[Baldchap]

I have a ply liined Vivaro. I didn't do the ply lining myself so have never seen behind it. The current setup of the van is such that I can't easily remove any of the ply without removing my double decker setup that I put in for taking full boards without unloading my gear.

My question is, assuming 6mm ply, what sort of screw lengths can I get away with when attaching things?

Obviously I could use grab adhesive, but it feels like a less clean solution.

Thanks in advance.