Underground Iron Farm

A hidden underground iron farm for three villagers and one zombie. Everything sits below ground with a glass spawn platform and the golems drop into lava over the hoppers, so nothing shows on the surface. It is cheap, under thirty slabs and a few buckets. Java only.

Spawnproof every cave around it. Underground means golems spawn in any dark cave nearby, and one live golem out there stops all new spawns. One builder slabbed everything within about 50 blocks before it worked.
It dies after one Minecraft day because the villagers never get to sleep. Dig a 3x4 hole in the wall behind the beds below the hoppers and spawnproof it with buttons, which breaks the line of sight long enough for them to hop into bed.
The fence gate burns from the lava. Replace it with three signs stacked off the side wall, or use nether wood.
Build at least 6 blocks underground. Golems can spawn 6 blocks above and 6 below a villager.
Make the spawn platform all glass except the strip of solid blocks down the middle. That strip is functional, not decoration.
Getting the zombie into the cage is the worst part of the build and he will take your gear. Name tag him so he cannot despawn afterwards.