How To Get The Most Renegades Coins

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..

That's still like 15-20 minutes per slate and making a Jett over and over is boring as pineapple.