Scalable high efficiency gunpowder farm

Creeper farm on Methodzz and SciCraft's spawning platforms, portal based, built over a perimeter or a lit up cave area. Creepers spawn on the platforms, ride a mob elevator to a nether bridge, and go through a portal into the killing chamber, where boats push them back out of the portal block. Trapdoors on the platforms keep everything but creepers from spawning. There are two versions of it. The 200000 gunpowder/h build is the one a single player can run on their own, and it is the same farm in every part except the killing chamber. The 1.2 million version replaces that chamber with a second player or a carpet bot doing the killing, so it only applies if you have a second account. Both are a showcase with schematics rather than a block by block tutorial. Java only, and on a server nobody can be in the nether more than 128 blocks from the mob elevator or the farm stops.

To get the chickens into the boats, block the portal with temporary solid blocks, place the chickens, remove the blocks, then light the portal. Otherwise they walk straight through.
On a server nobody else can be in the nether further than 128 blocks from the mob elevator. If someone is, you need a second player or a carpet bot AFK at the bridge.
One builder in 26.2 reports the boats in the spawning chamber drifting out of alignment after about five minutes, in single player, multiplayer and the world downloads alike. The creeper only version with the trapdoors is less affected.
Take the trapdoors off and it becomes a general mob farm, but then you get skeletons and zombies and no spiders.
17x17 chunks is the usual perimeter and the creator runs 21x21. You can skip the perimeter entirely and light every cave in range instead.