Fiberglass is one answer. I think I'd try to take a pc. of metal an old door or hood are good to use. If you don't have a leather bag filled with sand a old army buffle bag with sand can be used with a hammer to form the pc. Or go out in the yard and just beat it into the ground. Get it close then use a dolly and hammer. Don't try to bend it all at once it takes time . Once close weld it in and finish with bondo or fiberglass. The less filler you use the better.