The ends are separate pieces, so you could scratchbuild your own, if you wish. Unfortunately, the sides are one piece, so the bracing would be harder to change. In this case, I specifically needed the SP&S/GN cars for my BN, so made a master for Marshall, rather than scratchbuild more than one myself.
The major pieces are a metal floor casting, two resin sides, and two resin ends.
Today, it might be easier to make a master by 3D printing, rather than from styrene and wire. Anyone interested in designing it, and seeing if RLW, or someone else, will cast it? I agree, the SP version should sell.