I'm trying to think of a way to shrink the middle a bit. get a piece of corrurgated? cardboard.stick pins in the block around the edges first and then on the major seams from the outside towards the middle--until uou get to where it won't lay flat--then spray or dab it with water in the area where it won't lay
flat and let it air dry. See if that helps. I know that when I spray fabric with the steam iron it goes back to it's original size. this may help to flatten it. Anne