How To Get The Most Renegades Coins

I'm really hoping there will be more ways to farm them once Phantom comes out because I was banking on this event for Clean Slates.
Really regretting not farming them during the Bounty Hunter event. I disliked how the rewards from that were random though.

At level 15, you have 12 coins. Clean slates are 15, if I remember correctly. In your experience, what is the fastest way to get the remaining 3?

EDIT: I guess the only "fast" way is to level to 30 during the event, lol..