I got in contact with the guy who had designed the Berwick coupler boxes mentioned above and on sale at Shapeways. I asked him if he thought those boxes would fit my PS-4427 hoppers. He said no, but kindly (and totally surprising for me offered to design some that would). I guess he could use some for himself, and not did this out of kindness only. But who knows? Anyway, I am very grateful. We exchanged some proto and model photos, along with some model measurements, and he came up with this beautiful design:
The box is not specifically designed for the Protocraft Clouser coupler, since he wanted a more generic design. But I think I'll be able to come up with some way of mounting such a coupler in the box.
I have bought a six-pack of the coupler boxes and they arrived a few days ago, and in IRL they look like this:
Very good, don't you think?
The boxes are easily attached to the cars, using the original mounting screws and holes. They only need to be countersunk for the screws to not later interfere with the coupler shank. Here is one of the boxes test fitted on one of the cars.