In Sandbox, fly to the heighest possible point in the map. Look around till you can see the sun (not the "red light" on one side of the map) on the horizon, inbetween two Guardian towers. Turn right and face inbetween the next pair of Guardian towers (so the tower on the right of the sun is now the tower on the left). The skull is located outside of the map in this direction. If you are having trouble spotting it, try and use the zoom of a sniper rifle or beam rifle. There are several methods of obtaining the skull. You can use bubble shields, tube pieces and shield doors to form a tunnel, or you may fly over the skull and drop a teleporter exit so you can teleport to it directly. There are also several maps you can download on fileshare to gain easy access to the skull.