There are no mobs in the schematic: put an allay in every cell. Take off the top slab over the campfire, place the allay with a spawn egg, put the slab back. The campfires must be lit.
Allays on a campfire never die: they heal as fast as the fire hurts them. Throw a splash potion of infestation on top of the row: every burn has a 10% chance to release 1–2 silverfish, and they crawl to you in the trench. Stand in the trench in front of the cells and hit. One potion lasts 2:15, about 700 XP; 16 allays take 4 potions.
Build it from andesite, not cobblestone or stone: silverfish hide in those.
More in the article on the BeezzLand blog.
