Did you try the other clouds? They should behave different and go all the way to the horizon. What you are using is a method that overlays normal maps to generate the clouds, maybe that has something to do why they do not go all the way to the horizon, while the other clouds are just an image that scrolls.
To cover up that stuff in the distance you could make skyboxy with only clouds on the horizon or make white fog that only appears in the distance on the horizon.
But I don't really understand your problem, I use all methods of sky/clouds in my levels and the basic clouds and skybox go all the way to the horizon and the other clouds do not go all the way to the horizon, but fade out in the distance, it does not look that wrong to me and if you have some mountains etc in the distance you won't see the empty area at all.
Not really a problem with T3D but rather a problem im creating since i think im trying to do something its not meant to do, which is something you and i talked about for like a year ago. That is static cloud layer clouds for the purpose of getting clouds in the horizon that are effected by TOD. Didnt have time to dive into that then but by a coincidence that specific, unorthodox, way of making horizon clouds popped up in BeamNG some months after we discussed it and now im trying to improve the look of those.
Ive managed to figure out a way to get pictures of clouds, typically used for skyboxes, to look similar when used in cloud layers thus having them react to TOD/sky changes. Im really in a beta stage of this but what i do is i recolor the cloud texture only using normal map colors (blue/purpleish, redish, greenish) using a special tool in an online photo editor. If i was to use a common normal map tool i wouldnt get the good blending of the cloud shapes i now get.
Just figured out one way to solve the less visible clouds. Breaking it up in portions so that the clouds closest to the horizon is one texture, with higher exposure setting. Im already planning what benefits would come from using several textures for this but this would definitely be one.
The horizon clouds are meant to be one part of the sky clouds, the other is conventinal cloud layer clouds that move across the screen. One idea i did have was to have the horizon clouds rotate around the level, to bring some more dynamic to them, but im not that down with the codes to add that posibility myself. Not sure one would play each level long enough to even notice or care if they move or are static.