I spent today working on another wood chip car for the fleet. The BAR made a ton of these from old 40ft insulated boxcars and reefers. I pretty much make mine the same way that they did. Cut off the roof, remove the doors, extend the sides, move the brake wheel, and paint it up.
Unfortunately, I had an issue with the TruColor paint that I used to paint it. It popcorned on me leaving a very rough finish. However, I think with some careful weathering work, I can use it to simulate paint rusting from below.