Simple no build raid farm

The smallest raid farm in the library. One villager in a hole with a bed and a composter is the whole village, and there is no spawning platform at all: the raiders spawn naturally on the surface around you and walk in on their own. You drink an ominous bottle next to the villager, climb 28 blocks down a ladder shaft, and the raiders drop in after you and get held over your head by signs and water while you kill them. Lava on the side deals with the ravagers, and two boats sit there so that any vex that spawns climbs into a boat instead of hitting you. It costs about 60 blocks total and works from the first days of a world, which is the whole point of it. The tradeoff is speed, because everything depends on raiders walking to you.

On newer versions climb a few blocks back up the ladder to start the raid. The update made the 28 block drop too far for the raid to trigger from the bottom.
Drink the bottle next to the villager at the top and wait for the effect to change from bad omen to raid omen, then go down.
Use a level 3 bottle or higher. Below that you often get no totems at all, and the creator uses level 4 or 5.
The two boats are safety. A vex that spawns gets into a boat instead of attacking you.
Build it on open flat ground and light up the caves around it. Raiders that spawn in a cave or drop into a hole never arrive, which is why some builders wait forever for the last wave.
If there is a village nearby, break all the beds in it first or the raid goes there instead of to you.