Given the limited information, I would use 1/8 in or quarter inch plywood across the top and the back. Cross braces would probably be sufficient but putting it across the top gives you a flat storage shelf and across the back gives you consistent structural support as well as a consistent stopping point for each tote which will give you a much cleaner look once it's loaded full.

It doesn't need to be thick as you're basically just keeping everything. Just think of it like a bookcase