The Nissan Path

I'm thinking about building a cargo insert that'll slide into my pathfinder when I put all the seats down in the back to act like a pickup truck bed. I move a decent amount of dirt, rocks etc. for home improvement projects, and rather then throwing dirt in a garbage can or buying dirt in bags I was thinking of building this cargo liner so I can throw it right in there.

Have any of you done something similar? I know they make canvas "socks" for cargo bay in the pathfinder but I want something more rigid to act like a pickup truck bed.