Is it possible to use a cube map, and not have the “ripple” effect occur? It would appear as if the textured object was a window, through which you could see a skybox. This would be useful for placing a “sky” brush in a window, like in Quake 1.
Note that I am not talking about rendering an inside-out box. I am talking about rendering arbitrary geometry so that the space the polygons fill look as they would if they were part of a skybox.
Apparently, you are seeing a color gradient in a relatively small cubemap texture. When a gradient is displayed in a cubemap, you will see some form of ripple or pixellating effect unless the cubemap is large enough to allow the gradient colors to slowly shift from one value to another. Although that may be a good solution, that takes up memory. I suggest that you use some form of filtering to allow colors to shift from one value to another.