MC Edit is really cool, especially when you don't want to start over when creating a new world.

I have created several factories, that work in vanilla minecraft. This Obsidian house spawns skeletons at the back and creepers at the front. With the handles you can either open the Creeper door, so they come in and be shot by the skeletons, or you can open the other door to collect the albums. Be aware though, the skeletons might hit you with their arrows, so don't forget armour. So even should the creepers explode, they cannot destroy the factory.