Since it is steel then you could probably cut out the rust and if the edges are in good shape use it for a shelf angle to lay another bottom in it. If no edges are left then cut up the sides to get to good metal and use some galv gauge angles that will drop to bed level. You could use exterior grade metal stud track that you can get in different gauges and split them to make your angles. Raw steel metal should be primed with zinc rich galvanined primer to extend the service life. If you use a deformed corrugated shape for the bottom it will be lighter as the steel is stronger with a bent shape than a solid piece, weight-wise. Coat with a bed liner like Rustoleum . Leave some drain holes.
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