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Most of them have cross pieces that go across the width of the truck where the bed bolts are, and longitudal pieces that connect to those, much like a ladder. Then a metal top is welded on, or angle iron (steel or aluminum) is welded around the perimeter and wood planks dropped down.
Pretty easy to make if you can weld.
Though, I have to say, I absolutely LOVE the flatbeds that home depot has on their trucks. If you haven't seen it, its a bed thats a little shallower than a normal pickup bed, but the tailgate drops completely down, as do the sides, You can load it from either side or the back. Makes loading plywood and sheet rock a breeze if you're by yourself.