Instantiating an object randomly withing the boundaries of an object.

http://prntscr.com/fi8odo

Im trying to spawn an object called Asteroid in a random location within the boundaries of the green area with an interval. I can't figure out what i'm supposed to write in the Instantiation code. Could someone please help me?

 public class SpawnerScript : MonoBehaviour {
 
     public float SpawnRate = 3;
     private float NextSpawn;
 
 
     public GameObject Asteroid;
 
 
     // Use this for initialization
     void Start () {
 
     }
     
     // Update is called once per frame
     void Update () {
 
 
 
         if (Time.time > NextSpawn) 
         {
             NextSpawn = Time.time + SpawnRate;
             Instantiate(Asteroid,???
         }
 
     }
 }