Stackable cat creeper farm

The original Chapman creeper farm, and the design most later cat creeper farms are built from. Creepers spawn in a small dark box, two cats sitting in the middle scare them onto trapdoors around a central hole, and they drop into lava at the bottom. Trapdoors on the ceiling stop zombies and skeletons spawning, because creepers are shorter than both, and carpet on the floor stops spiders. That is the whole trick, and it costs four stacks of blocks and two of trapdoors. It stacks, the roof of one farm becoming the floor of the next. Java only, and the AFK spot has to sit exactly 120 blocks above the chest or nothing spawns at all.

AFK exactly 120 blocks above the chest. Nearly every report of the farm producing nothing is this or unlit caves underneath.
Light every cave and mineshaft under the build. One builder spent an hour and a half on it and says that was what made the difference.
Spiders still spawn in a 3x6 strip through the middle, because the six trapdoors make the hole read as solid floor to the spawning code. A thirteenth and fourteenth carpet next to the middle trapdoors closes it.
The two blocks over the lava matter for the rate, do not leave them out.
If the gunpowder is burning up, put a double chest under the lava so the creepers touch it without sinking in, or swap the lava for magma blocks. That is the creator's own fix.
It stacks by turning the roof of one farm into the floor of the next. Thirteen layers with 26 cats is the largest anyone reports.