Sunken twelve villager iron farm

An early game iron farm for twelve villagers and two zombies, dug into flat ground. The villagers sit in a sunken chamber around two zombies in boats, golems spawn in the middle and wash into lava over the hoppers. The materials are cheap, mostly slabs and building blocks, but it needs a large flat area spawnproofed all around it. Java only.

Let the villagers sleep before you bring the zombies in. One builder posted that it does not work, then came back to say that was the only thing wrong.
If golems spawn around the farm instead of in the water, spawnproof further out. The creator's own answer is to make the spawnproofed area larger, and the 15 blocks shown is a minimum.
Digging the whole chamber one or two blocks deeper removes the need to path block the top at all, since the villagers do not stand on the floor.
Build about 100 blocks from the nearest village. If you build closer, break every bed and workstation in that village first.
The soul sand is there to push the boat, not to make a bubble column.
If the iron burns instead of reaching the chest, check the hoppers actually run into the chests and that only the middle lava source is placed.