[lunarhighway]

you might look into computer casings, people throw out their 'old' computer after a few years... the lid is usually a U shaped metal sheet so unfolding the side 90° will get you a flat sheet.

might not be suitable for everything but it could make an under engine cover or something.

i'd advice looking for thin aluminum however, it'll be cheaper, lighter, and folding the edges up at a 90° angle will increas the strenght and regidity very much