Simple Underground chamber iron farm

A small iron farm for three villagers and one zombie. The villagers sit in a chamber below the surface with the zombie in sight, golems spawn on the path blocked platform above and wash down into a lava blade over the hoppers. Cheap and quick, about 60 blocks in total. Java only.

Panicked villagers never sleep, and a villager that has not slept in the last 20 minutes stops spawning golems, so the farm dies after roughly 20 minutes. Add something that breaks their line of sight to the zombie at night, pistons with sand on a repeater loop or a daylight sensor driving trapdoors.
Golems spawn outside the farm. Eight blocks of path is not enough, shovel 9 or 10 blocks out on every side.
Golems get stuck in the walls. Widen the water channel from 3 blocks to 4 and extend the lava the same way, or place signs on the walls.
If one villager will not sleep, turn the bed round, or give the chamber 2 more blocks on each side.
As of 26.2 golems can spawn on path blocks, so the path on its own no longer spawnproofs the area.
The villagers have to sleep while you are inside simulation distance. Sleep does not register if you are away, and then no golems spawn.