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: