I didn't read the entire thread so I'm not sure this has been suggested. But after a litho course in college, I started using gum tape to tape down my FB prints onto a piece of plywood. I usually leave enough border so that the image isn't affected and the paper dries as flat as the board I'm using. Leave until air dry.

Good quality gum tape has to be cut off though, while lower quality seems to just peel off of the border of the paper without remnants.