Wide print slightly warped - suggestions?  

Wide print slightly warped - suggestions?

I printed a case that is about 210x170mm and only about 9mm high (6+ hours).

After I removed it from the printer, I noticed that it wasn't flat. Not the usual lifting on the corners, but the middle of the edges were curved up.

Adhesion to the bed was great - before I removed it, the entire print was flat and fully adhered. Also, I didn't remove it until everything was cooled down (and even if I removed it early, the bending would be in the other direction).

The case is printed flat (NOTE the wavy vertical parts are dead straight - the wave is a camera aberration):

and then hinges into a closed case (no camera aberation here):

Holding the 2 main parts (unassembled) with the base surfaces against each other shows the warp too:

Any insight would be appreciated. Thanks.