Chess board update.

I glued up 4 of the "two strip" sub assemblies. When set, an hour later, I unclamped and scraped off excess glue. The joints are perfect...no gaps. I took these two "4 strip" panels inside. This morning I looked and the one that was laying on top had formed a cup. Ugh! The bottom one is still flat. Now I need to figure out what to do before continuing. Perhaps I made these too thin.