Jon,
The prototype screens are unique, and the etched part is based on the prototype. They are designed specifically to fit the windows of the KV model and won't be available separately.
For your B&M "New Image" caboose screens, I recommend Hasegawa Trypart modeling mesh. It's only marketed in Japan, but you can Google for U.S. sources. As with the KV screens, the Trypart is thin etched stainless steel. The mesh is super-fine and there are various styles - square, diamond and hexagonal. It's exactly what you are looking for. Based on your prototype photo, you want the diamond mesh (item# PA-21).