I have a hankering to kitbash the PRR H31B composite hopper. As far as I know the only model produced was by PSC in brass. The Weaver and Atlas versions of a composite war emergency hopper have a very different outside rib braces, that would need extensive work.
I was thinking of using a plastic slab sided USRA style hopper as a starting point, unless anyone has another suggestion. Here is a photo of the car.