Eidolon

The image above tells how the permanent mob spawn work,
 
This is the script for the permanent mob spawn in an area,
mapname,x,y,x1,y1,
 
x,y, is the red dot
 
while
 
x1,y1 is the distance from the red dot to the yellow dot.
 
So, in this case,
 
The script will be
 
mapname,3,3,2,2  
So the monsters will spawn inside the created figure.

Something like this?