I want make a box colleder for top, botton left and roght in c# but dont work

my boxs colliders don't work I dont know the reason. ok well explained, I created 4 objects with their respective boxcollider2d, now I want that code add to up, down, left and right of the object camera MainCamara , that the boxcollider2d are around my MainCamera object. But the truth nothing happens now code review and has no error in console or anything like prefectamente runs the game.

this is my code:

 using UnityEngine;
 using System.Collections;
 
 
 public class GameSetup : MonoBehaviour
 {
     //declaramos camara
 
     public Camera MainCamara;
 
     //declaramos variables de box collider
     public BoxCollider2D topWall;
     public BoxCollider2D bottonWall;
     public BoxCollider2D leftWall;
     public BoxCollider2D rightWall;
     //declaramos transformaciones para los jugadores
 
     public Transform Player01;
     public Transform Player02;
     //inicializamos codigo
     void update()
     {
         topWall.size = new Vector2(MainCamara.ScreenToWorldPoint( new Vector3 (Screen.width * 2f, 0f, 0f)).x, 1f);
         topWall.center = new Vector2(0f, MainCamara.ScreenToWorldPoint(new Vector3(0f, Screen.height, 0f))  .y + 0.5f);
 
         bottonWall.size = new Vector2(MainCamara.ScreenToWorldPoint(new Vector3(Screen.width * 2f, 0f, 0f)).x, 1f);
         bottonWall.center = new Vector2(0f, MainCamara.ScreenToWorldPoint(new Vector3(0f, 0f, 0f)).y - 0.5f);
 
         leftWall.size = new Vector2(1f, MainCamara.ScreenToWorldPoint(new Vector3(0f, Screen.height * 2f, 0f)).y);
         leftWall.center = new Vector2(MainCamara.ScreenToWorldPoint(new Vector3(0f, 0f, 0f)).x - 0.5f, 0f);
 
         rightWall.size = new Vector2(1f, MainCamara.ScreenToWorldPoint(new Vector3(0f, Screen.height * 2f, 0f)).y);
         rightWall.center = new Vector2(MainCamara.ScreenToWorldPoint(new Vector3(Screen.width, 0f, 0f)).x + 0.5f, 1f);
 
         Vector3 newpos = Player01.position;
         newpos.x = MainCamara.ScreenToWorldPoint(new Vector3(75f, 0f, 0f)).x;
         Player01.position = newpos;
    
         Vector3 newpos2 = Player02.position;
         newpos2.x = MainCamara.ScreenToWorldPoint(new Vector3(Screen.width - 75f, 0f, 0f)).x;
         Player02.position = newpos2;
     }
 }

this is what I want a square with boxcollider2d:

http://www.subirimagenes.net/i/141009033817469385.png

but not wrong in my script does not work me, and I have this:

http://www.subirimagenes.net/i/141009033820261585.png