Simple Six villager drop shaft iron farm

A compact iron farm for six villagers and one zombie. The zombie sits in a boat below the villager platform, golems spawn above him and fall down a shaft into lava over the hoppers. It takes about 15 minutes to build, needs no perimeter, and it stacks if you want more than one layer. Java only.

The boat placement matters. Off centre it catches the falling golem, or it drifts out from under the water block and the sun kills the zombie during the day. Making the zombie's roof a 3x3 of bottom slabs instead of the single slab fixes the sun.
Use glass for the temporary blocks while you bring the zombie in, otherwise a golem drops in and kills him before you are done.
If golems spawn on the ground instead of the platform, put trapdoors around the edge of the hole and open them. The golems read them as solid and walk in on their own.
Check the villagers have not already claimed a bed somewhere else before you move them in. A villager linked to another bed never sleeps and never spawns golems.
Make the drop deeper, or raise the top part by about 10 blocks, so golems leave villager detection range sooner. One builder measured 630 iron/h after that change.
Villagers shove each other off the platform. Place blocks above the beds so they cannot run over the top.