Ok, before you begin you'll need 1000 460 scrap.
Go into your inventory and scroll through to the weapons tab.
Filter for rare (blue) weapons as at time of writing this solution there are no common series 3 content. Find the UIR weapons. These should be 100 scrap each. MAKE SURE IT'S NOT THE UIR ELITE WEAPONS! These are 600 scrap each.
Filter for common weapon skins & find the biker skins, I can find 9 of these. Buy 1 of each so that you have no duplicates.
Filter for rare weapon skins & find a UIR weapon (NOT A UIR ELITE!) buy 1 of these.
EDIT: Some folks are having to restart the game for this to pop.
EDIT: Thanks to Trombonafide for a cheaper update to this solution.