How to Make a Killbox in RimWorld 1.6: Layout, Corner Cover and Turret Spacing

How to build a killbox in RimWorld 1.6 that raiders still walk into. Wall the base and leave one gap, run a one-tile tunnel with two 90-degree turns, and open it straight onto your firing line. In 1.6 raiders can stop and shoot from sandbags and barricades, so leave the exit bare: a raider in the doorway gets 0% wall cover from shooters within about 25 degrees of straight ahead, 30% at 45 degrees, and 75% from beside the opening. Also covers lane depth by weapon range (chain shotgun 12.9 tiles, heavy SMG 22.9, assault rifle 30.9), a spike trap and fence lane for colonies without turrets, mini-turret spacing (3.9-tile blast, keep them 4 apart), the autocannon's 5.9-tile blast and 8.9-tile minimum range, four reasons raiders won't come in, and the raids that skip a killbox.