If I'm reading you correctly, I think there are a couple different ways to skin this cat. It sounds like you want flat panel .dxf drawings to cut the flats for each side. The part is too big to form from one sheet, so you need to cut each piece separately and weld them up.
The easiest way to me would be to cut an extruded boss that represents the inside negative of the shape you want. Think of this like the inside of a present, and your sheet metal will be the wrapping paper. Start with a simple extruded rectangle, then make cuts on the sides using the extruded cut tool. Draw the boss sketch on the top plane, extrude up, and then take cuts from whatever planes you need to make your shape.
Once you have the shape you can either shell it or simply export the side shapes as a .dxf. You can then take the .dxfs and create 4 individual flat parts and mate them in an assembly.
If you need me to help send me a dimensioned sketch and I can take a crack at drawing it for you.