How can I loop objects with Polygoncollider2D attached to them?

I have a background looper script which is looping all the objects in the scene with the Boxcollider2D attached to them. I have now Polygoncollider2D objects in the scene but they don't loop due to the fact they don't have the box collider attached to them. I use the following script to make the box collider loop in the background looper;

 using UnityEngine;
 using System.Collections;
 
 public class BGLooper : MonoBehaviour {
 
     int numBGPanels = 4;
 
     void OnTriggerEnter2D(Collider2D collider) {
         Debug.Log ("Triggered: " + collider.name);
 
         float widthofBGObject = ((BoxCollider2D)collider).size.y;
 
         Vector3 pos = collider.transform.position;
 
         pos.y += widthofBGObject * numBGPanels;
 
         collider.transform.position = pos;
     }
 }
 

The problem is located in the part, describing its size (.size.y) as can be seen below:

I know Polygon colliders work with points instead of size but I wasn't able to implement it in this script. If u have any suggestions or recommendations please let me know :)