Simply gives you a custom sign. Place it in a 4x4 clear area, zombie will spawn 2 blocks forward, and 2 blocks right from the sign when right clicked. "Zombie" can be changed to any other mob.
The command
Note: Minify the command to avoid issues :)
Created: Thu, 27 Oct 2016 19:09:13, Updated: Sat, 13 May 2017 23:12:06, Views: 24