I'm sure this has been asked several times. I did search the forum and couldn't find what I was looking for.
What can I do to get rid of the lines in the corners of the skybox? I have used the surface properties editor and the manual texture mover. Right now, it's the best that I can get it. Any movement in either direction and it's way worse. This is the main reason I removed this skybox for my Nali temple map and just went with a stock skybox.
From my experience UT99 was not built with these cubemap skyboxes in mind, so there is no official support for them which would solve all issues. Usually a game engine has a dedicated option for this where the engine takes care of the border. But no such things in unreal engine 1 so consider making an unreal style skybox which contains geometry, light and a few layers of clouds/starts, maybe a sun sprite.
If you still want to go with this cubemap skybox then best thing you can do is to stretch&pan it a bit, then test it.
Some people might tell you to turn off linear interpolation but don't do it! You definitely want interpolation on your skybox, otherwise it stands out too much when you see it between level geometry which has interpolation enabled.
Just to make things extra interesting each renderer renders a bit differently, I've seen some of them apply a slight 1 texel V-pan, so you'd better test with multiple renderers and repeat the process of stretching/panning.
You can also increase the DrawScale for the texture, this will give you a finer control when panning, with a DrawScale of 4.0 then your 1U pan becomes a 0.25U pan, so you can get some much needed precision.
PS: if you send me the textures, I can convert them for you to 256 colors at higher quality
