Quake3World

Is there anything to do about textures that wraps like 1px in the edge, so that a part of the bottom is visible at the top? Should I 'preemptive-wrap' the texture by 1px but the other way or are there some shadervalue that can be used? This woulden't be a problem, but now the texture is being used in a _skyportal x300, and, it looks pretty bad since of the large scaling.

As you suspected, you basically need to stretch the mapping out so there is one texel beyond each edge to stop the wrapping. Assuming you can use some sort of modelling program, converting the skyportal geometry to a model and fixing UV issues that way will be the best thing to do.

You'll probably want to make sure the shader doesn't allow mipmapping too.