Camera custom projection matrix

Hi
I want to create a scene where all the elements of the scene cloned and rendered at a smaller size at the center.
Here a (broken) screen of what i want to do.

I don't want to physicaly clone the gameobjects, because the player can interact with some objects, and the interaction have exactly the same influence on the cloned area.

For that i use a 2nd camera placed in a place that simulate the render of the same room, but smaller.

But the render is not at the right position because it's deeper on the z buffer.
I have see that i can edit the projection matrix of the camera to allow to draw at an other height on the z buffer.
https://answers.unity.com/questions/1359718/what-do-the-values-in-the-matrix4x4-for-cameraproj.html
I have tried to modity the values of m22 and m23 to simulate the near and far plan of the main camera, but it don't work well.
Here the code of the clones cameras positions and the modification of the projection matrix.

Code (CSharp):

1.  
2. class CameraInfo
3.     {
4.         public float scale;
5.         public Transform transform;
6.         public Camera camera;
7.  
8.         public float near;
9.         public float far;
10.     }
11.  
12. class GetCameraEvent
13. {
14.     public Camera camera = null;
15. }
16.  
17. void ProcessCameras()
18.     {
19.         GetCameraEvent camera = new GetCameraEvent();
20.         Event<GetCameraEvent>.Broadcast(camera); //main camera
21.  
22.         Vector3 center = transform.position;
23.         Vector3 pos = camera.camera.transform.position;
24.         Vector3 forward = camera.camera.transform.forward;
25.         Quaternion rotation = camera.camera.transform.rotation;
26.  
27.         float near = camera.camera.nearClipPlane;
28.         float far = camera.camera.farClipPlane;
29.  
30. //for all cloned camera
31.         foreach (var c in m_cameras)
32.         {
33.             c.transform.rotation = rotation;
34.  
35.             Vector3 zero = -center / c.scale + center;
36.  
37.             Vector3 cameraPos = pos / c.scale + zero;
38.             c.transform.position = cameraPos;
39.  
40. //we use the same base projection matrix than the main camera
41.             var mat = camera.camera.projectionMatrix;
42.  
43.             var dir = Vector3.Project(pos - cameraPos, forward);
44.             float dist = dir.magnitude;
45.             if (Vector3.Dot(dir, forward) < 0)
46.                 dist *= -1;
47.  
48. //near and far plan on the same plan than the main camera
49.             float currentNear = near + dist;
50.             float currentFar = far + dist;
51.  
52.             c.near = currentNear;
53.             c.far = currentFar;
54.  
55. //change matrix for z buffer
56.             mat.m22 = -(currentFar + currentNear) / (currentFar - currentNear);
57.             mat.m23 = -(2 * currentFar * currentNear) / (currentFar - currentNear);
58.  
59.             c.camera.projectionMatrix = mat;
60.         }
61.     }
62.  

Is there a way to do what i want without cloning every gameobject multiple times ?

Sorry for my bad english, it's not my native langage.
Thanks for your help

Nico