The way I remember seeing it done was to insert the rebar completely through the 2x8s bolted from the outside. On long beds would probably need 2 or3 ties. It ties the 2x8 together preventing the sides bulging out and therefore sliding. Might have to displace some dirt to find the exit hole. Best to insert them before filling, I suppose. Hell, it might not even work lol

I'm still debating on the metal too, but might go prefab