Re-uploaded, didn't mean to place this originally in skins :)
By no means is this 100% perfect, I tried to center this as best as I could due to the way portal renders it's crosshair. Not sure how relevant Portal 1 skins are anymore so I will work on a Portal 2 replacement as well.
I created this crosshair with the intentions of the user having 1920x1080 resolution. I have not tested this with anything higher than 1080p, so experiences may vary.
The development of this crosshair was influenced by the default Half-Life 2 HUD style crosshair and Portal: The Flash Version crosshair.