Thousands of semi trailers are more or less permanently parked for storage.
As someone who pulls them for a living: since the goal is the lightest weight, they are built as flimsy as structurally possible. (Seriously: if the builders could, they'd use aluminum foil sides.) Sides are sometimes fiberglass, more often sheetmetal with vertical supports about every foot. Roofs are translucent fiberglass-type material or sheetmetal.
