How do you un-warp cardboard?

I drive a Honda Civic and in the trunk is a flap about 4 feet by 4 feet that covers the spare tire. A couple weeks ago, I had a container of liquid in my trunk and it leaked onto the flap. I assume the flap, while covered in the same carpeting of the rest of the trunk, is made of cardboard because it has warped and now bends about 10 inches off the bed of the trunk. I am thinking about resoaking the board and placing weights on it to help it reflatten but I'm not sure this will work. Any suggestions???