Before you panic try taping (packing, masking, duct) the deck down where it does fit, then see if you can work the gap closed. First bridge the gap at the widest point with your hand and squeeze, if it closes you should be in good shape, if it gets smaller but fails to close just move toward a narrower section of the gap and try again. As you get each section of the gap closed secure it with tape, and alternate sides as you go. If this doesn't work I would move on to a tougher clamp, like a band clamp for glueing chairs or picture frames. Go easy if you decide to use a clamp as it will apply a lot more force than you may realize. If everything aligned while it was on the forms it should go back to that shape with enough persuasion. If for some reason it won't then there is always the plane and sandpaper to fall back on.

Tom