How do i calculate the outer bounds???

Hello so i have a script on duplicating Vector3[](newVerts) but i would like to know how can i only duplicate the outer bounds vertices(newVerts) of the object, like this in the picture below?

Would anyone know how to do this, i'll be very pleased if some could help me out with this?

Here is my duplication code.

 public class Extruder : MonoBehaviour
 {
     public float rotAngle;
     public int stretch = 5;
 
     public MeshFilter mf;
     private Vector3[] origVerts;
     private Vector3[] newVerts;
     float xStretchDistance = 0f;
     float zStretchDistance = 1f;
     float yStretchDistance = 0f;
 
     void Start()
     {
         mf = GetComponent<MeshFilter>();
 
         Generate();
     }
 
     void Generate()
     {
         origVerts = mf.mesh.vertices;
         newVerts = new Vector3[origVerts.Length * stretch];
 
         for (int i = 0; i < origVerts.Length; i++)
         {
             for (int j = 0; j < stretch; j++)
             {
                 float newX = origVerts[i].x + xStretchDistance * j;
                 float newY = origVerts[i].y + yStretchDistance * j;
                 float newZ = origVerts[i].z + zStretchDistance * j;
                 Vector3 pos = new Vector3(this.transform.position.x + newX, this.transform.position.y + newY, this.transform.position.z + newZ);
                 newVerts[j + i * stretch] = pos;
             }
         }
 
         Mesh newMesh = new Mesh();
         newMesh.vertices = newVerts;
         mf.mesh = newMesh;
     }
     void OnDrawGizmos()
     {
         Gizmos.color = Color.white;
         for (int i = 0; i < newVerts.Length; i++)
         {
             Gizmos.DrawSphere(newVerts[i], 0.1f);
         }
         Gizmos.color = Color.green;
         for (int i = 0; i < origVerts.Length; i++)
         {
             Gizmos.DrawSphere(origVerts[i], 0.2f);
         }
     }
 }