its easier than i thought. put a camera into it with 18 / 36 / 90 for the lense values and than render 4 images at 90° steps (1..4) , and than 0° pan and 90 up and 0pan and -90 down (5, 6). imagesizes should be 256x256 512x512 or higher. put them together as a cubemap following order:

1 (pan 0) 2 (pan 90) 3 (pan 180) 4 (pan 270) 5 (pan 0 tilt -90) and 6 (pan 0, tilt 90)
you need to rotate the 5th image (down) 90° counterclockwise and the 6th (up) 90° clockwise ,
save as skyname+6.tga and ready to go

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