Instructions: Extract the archive in your GSC folder. Better remove your old skins (that you have used in different leagues or for testing the skin) so you don't accidentally take one of them. Because I changed the pitgroups, the files are a little bit different than what you have sent me. You must take a skin from the skinpack when you join the race server. When you are unhappy with your skin, then take a free RD car, but never make changes to the skinpack files. This will cause mismatches and you might be excluded from the league.
Download: check updated file below
Thanks to Reiza, RD and every single skinner!