i found quite a nice way to do it, place in your map at the origin an info_target with the name skybox_origin, and place at the edges of your (where you want to see the skybox coming) also a few info_target's with the name skybox_edge, then go to the enitity report and select everything where the targetname starts with skybox_, then press ctrl+M, select scale and set the x y and z values to 0,0625 (1/16) if your skycamera is set to scale 16.
now place your selection of info_targets with skybox_origin at the same place of the sky camera, and you'll se exactly where your map edges are, so now you're able to create your skybox to fit perfectly around your map
