Hello,
I’ve created an internal request to make FFogUniformParameters publicly accessible, but until then you’ll need to make a suitable engine modification to expose the structure and maintain the change when you integrate future versions of the engine. Alternately, you can do what the Water plugin does in Water.build.cs and gain access to the Renderer Private folder with:
PrivateIncludePaths.AddRange( new string[] { System.IO.Path.Combine(GetModuleDirectory("Renderer"), "Private"), } );