File:Mob Spawner spawning blocks diamond Bedrock Edition - Skeleton, Zombie, Creeper.svg

Summary
Created using 2 command blocks on a fast timer:


 * /execute at @e [family=monster] run setblock ~ ~ ~ stained_glass
 * /kill @e [family=monster]

The top spawning layer has different types of blocks above it to illustrate spawning requirements:
 * Zombies and Skeletons are nearly 2 blocks tall.
 * Creepers are somewhat shorter.

Licensing

 * Minecraft screenshot should be covered under fair use for minimal illustrative/educational purposes.