How to force all spawner mobs to spawn on one platform?

I want to make a mob drop trap with spawners in the Nether, and as it's the Nether I can't use water, so I want to have them spawn on one platform which retracts with pistons every so often. I tried to cover the area around the platform with cobblestone slabs, but the mobs were just spawning on the slabs. How do I make sure they spawn on that platform only (with the least amount of blocks)?

I recreated the platform with the spawners in a superflat world, the gold blocks represent the platform where the mobs should spawn:

minecraft-java-edition

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Quick Answer about "How to force all spawner mobs to spawn on one platform?"

Fill the area around with blocks, or.

Fill the ground and adjacent walls with torches. Place them everywhere, on each block you can, except the platform and it's close surroundings.

How do you force mobs to spawn in Minecraft?

Use /gamerule doMobSpawning false and then the mobs will spawn only from spawners.

How do you make mobs spawn only from spawners?

Spawners can be picked up with silk touch, but they can only be reactivated on top off a full tier beacon piramid.

Learn The Minecraft Mob Spawning Algorithm in 5 Minutes

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Images: Specna Arms, Miguel Á. Padriñán, Ray Bilcliff, Faruk Tokluoğlu