Plywood is likely overkill, unless you have very heavy items in your box. I'm using tempered hard-board (Masonite) riveted to some thin aluminum angle stock in my box. I used backing washers on the rivets that touch wood, and it's working great. Stong, light, and durable, and just a little bit flexible if I need to "stuff" something into one of the shelves. (Like putting in the white mini-drawers on the right, they fit, but to get them in under the lip of the box requires the shelf to flex just a bit until you clear the edge...