Scarecrow Hat

Scarecrows spawn during the Pumpkin Moon and nowhere else, so the set needs the event running and a night spent clearing waves. How likely a piece is to drop depends on how deep into the night you have pushed, which changes the strategy completely: restarting at the first wave over and over is the worst way to collect the outfit, and holding out as long as your arena allows is the best. Scarecrows keep spawning through the later waves, so advancing never cuts off your access to them.

Hat, shirt and legs are cosmetic and belong in the social slots. Each piece rolls on its own, so a complete costume rarely comes together in one night, and the Scarecrow shows up often enough that you will be sitting on duplicates before the last piece appears. You are already running the moon for weapons and trophies, so the set costs nothing beyond inventory space.