You may have luck spraying it down with water and weighing it down. I had a 3/4" sheet that leaned against my wall over the winter (a couple of winters ago) and warped a good 3" - 4" . I cut the pieces for my project out and nailed it up thinking the backing would square it up...no luck still had one end wanting to raise about 1 - 1 1/2" off the ground. Ended up spraying the tops and bottoms of the warped pieces with a spray bottle of warm water several times over about 2 hours, then setting 2 sacks of concrete on each end to pull them straight. After a couple of days of drying they held straight. Worth a shot.
To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault