Simple AFK mob farm designs for Terraria?

Easiest one that works: a long flat strip of ground, you standing in a small box in the middle, and a shallow layer of lava on both sides (one block deep so drops don't burn, or use wire and dart traps if you're worried about losing items). Mobs walk in, die, items pile up. Add a Battle Potion and a Water Candle to raise spawns, and build it far enough from your town that NPCs don't kill the spawn rate. During a Blood Moon it fills up stupidly fast.