Actually, pretty thin metal works fine. Cant remember the actual guage at the moment, but basically one guage higher than flimsy flashing; still very thin stuff. The trick here is in the "sandwiching" of the metal. If the wood on each side is really tightly put together, that means the metal would have to bow straight up and down to start deflecting, which is really really tough to do. Think in terms of a 6 or 8 inch wide piece of flashing metal; easy to flop around side to side, yes, but just try holding it flat and making it bend top to bottom. It cannot happen without wrinkling.