GUI3DCameraFitter

using UnityEngine;
using UnityEngine.UI;
using System.Collections;

[ExecuteInEditMode]

public class GUI3DCameraFitter : MonoBehaviour {

    public RectTransform targetRectTransform;   // Target RectTransform for size
    public Camera originCamera;                 // Camera that renders the whole UI
    public Camera targetCamera;                 // Camera to overlay over the RectTransform

    void Update () {

        // All the items are required for this script to work
        if (!targetRectTransform || !originCamera || !targetCamera)
            return;

        Vector3[] corners = new Vector3[4];
        targetRectTransform.GetWorldCorners(corners);

        // Corner reading is in this order:
        //
        //  1 ------- 2
        //  |    +    |
        //  0 ------- 3
        // SX: left | DX: right. Sorry, this wasn't in english before!

        Vector3 downSX = originCamera.WorldToScreenPoint(new Vector3(corners[0].x,corners[0].y,0));
        Vector3 upSX = originCamera.WorldToScreenPoint(new Vector3(corners[1].x,corners[1].y,0));
        Vector3 upDX = originCamera.WorldToScreenPoint(new Vector3(corners[2].x,corners[2].y,0));
        Vector3 downDX = originCamera.WorldToScreenPoint(new Vector3(corners[3].x,corners[3].y,0));

        Rect finalRect = new Rect();
        finalRect.x = downSX.x/Screen.width;
        finalRect.y = downSX.y/Screen.height;
        finalRect.width = (upDX.x - upSX.x)/Screen.width;
        finalRect.height = (upDX.y - downDX.y)/Screen.height;
        targetCamera.rect = finalRect;

        targetCamera.orthographicSize = (corners[2].y - corners[3].y) /2f;
    }
}