カメラを調整して、画面のアスペクト比を固定にし、任意の方向に画面を寄せます。

using UnityEngine;
using System.Collections;
 
public class CameraPortRect : MonoBehaviour
{
    //-------------------------------------------------------------
    //! @name	画面の位置関係.
    //-------------------------------------------------------------
    //@{
    public enum ALIGN
    {
        CENTER = 0, //!< 中央寄せ.
        UP,         //!< 上寄せ.
        DOWN,       //!< 下寄せ.
        LEFT,       //!< 左寄せ.
        RIGTH,      //!< 右寄せ.
    };
    //@}
 
    //-------------------------------------------------------------
    //! @name	メンバー変数.
    //-------------------------------------------------------------
    //@{
    public float baseHeight = 960.0f;           //!< 基準となるスクリーン高さ.
    public float baseWidth = 640.0f;            //!< 基準となるスクリーン幅.
    public ALIGN screenAligh = ALIGN.CENTER;    //!< 画面寄せ
    //@}
 
    //-------------------------------------------------------------
    //! 開始.
    //-------------------------------------------------------------
    void Start()
    {
        Camera camera = gameObject.GetComponent<Camera>();
        float baseAspect = baseHeight / baseWidth;
        float nowAspect = (float)Screen.height / (float)Screen.width;
        float changeAspect;
 
        if( baseAspect > nowAspect )
        {
            // 横基準
            changeAspect = nowAspect / baseAspect;
 
            switch (screenAligh)
            {
            case ALIGN.CENTER:
                camera.rect = new Rect((1 - changeAspect) * 0.5f, 0, changeAspect, 1);
                break;
            case ALIGN.LEFT:
                camera.rect = new Rect((1 - changeAspect) * 0.0f, 0, changeAspect, 1);
                break;
            case ALIGN.RIGTH:
                camera.rect = new Rect((1 - changeAspect) * 1.0f, 0, changeAspect, 1);
                break;
            }
        }
        else
        {
            // 縦基準
            changeAspect = baseAspect / nowAspect;
 
            switch (screenAligh)
            {
            case ALIGN.CENTER:
                camera.rect = new Rect(0, (1 - changeAspect) * 0.5f, 1, changeAspect);
                break;
            case ALIGN.UP:
                camera.rect = new Rect(0, (1 - changeAspect) * 1.0f, 1, changeAspect);
                break;
            case ALIGN.DOWN:
                camera.rect = new Rect(0, (1 - changeAspect) * 0.0f, 1, changeAspect);
                break;
            }
        }
    }
}

このコードをカメラのGameObjectに割り当てれば良いです。