A position behind a moving gamobject but still inside another gameobject

Hi,

i'm trying to find a random position in 3d world space behind an AI driven character. The problem is, the position have to be inside the boundarys of another gameobject(cube).

What i have so far is the random position inside the whole cube, but not only behind the AI.

 public GameObject WaterCube;//the water itself
 private BoxCollider Water;//get the size of the water
 public static Vector3 Tank;//save the size of the collider here
 
 void Start () 
     {
         Water = WaterCube.GetComponent<BoxCollider>();
         Tank = Water.bounds.size-new Vector3(1,1,1);//decrease the bounds by 1, so nothing would be in the glass
         Water.enabled = false;//disable collider because of weird effects might happening
     }
 
 void OnTriggerEnter(Collider other)
     {
         if (other.gameObject.tag == "Glas")
         {
             if (!turning)
             {
                 //recalculate newGoalPos
                 newGoalPos = UpdateGoalPos();
             }
             turning = true;
         }
     }
 
 Vector3 UpdateGoalPos()
     {
         goalPos = new Vector3(Random.Range(WaterCube.transform.position.x-Tank.x/2,WaterCube.transform.position.x+Tank.x/2), 
             Random.Range(WaterCube.transform.position.y-Tank.y/2,WaterCube.transform.position.y+Tank.y/2), 
             Random.Range(WaterCube.transform.position.z-Tank.z/2,WaterCube.transform.position.z+Tank.z/2));
         //Instantiate(WayPoint, goalPos, Quaternion.identity);
 
         return goalPos;
     }

I hope you have an idea how i can approach this. Thanks in advance