1/2" plywood, glued and nailed together + bins from Walmart, fitting perfectly. A long bin like this and a half length bin fit per side. I have one side with a long bin holding rope, tools, tarp, misc and the short bin holds my camp kitchen stuff. The other side gets no bins, but gets shovel, folding saw, hatchet, axles and folding chairs..
simple cheap. bolt it down though so its solid in case of an emergency. (Gotta say it for safety, right)
mine is painted the same color as the bodywork back there, and when half covered with typical rover junk, you cant even tell that there is a shelf in there.
Then only thing I notice, is that the contents of the bin get HOT and subsequently damp from condensation. not sure if this is a my rover thing or an all rovers thing, but just wanted to mention it. Exhaust pipes underneath I guess?
~Steve