Look into www.millsupply.com or www.keystone.com
They might have something. I in my many years have used from: a 55 gallon
drum section, then hammer and dollied the sides, and made side panels to the
Regular automotive sheetmetal, and formed the trough section, hammer and
dollied the sides for rigidity, and made side panels, to the floor.
Used old hoods, doors and roofs, to fabricate these parts. Lots of wagons
needed them in my day!