Back when I did the EBT boxcar kit, I was also working on a similar kit for the 3-bay hoppers. These would have been acrylic cut parts for the body and frame, and Plastruct shapes for the rest. I never got a good design that I liked, and then AMS came out with their 3 bays so I was less inclined to finish it.
But, I still have a bunch of prototype parts. And they can be yours, for free (well, shipping), with a catch. If you end up figuring out how to build them, I would like one or two of them. I have enough cut sides to make 14 cars.