Need Help Spawning Cubes Constantly In Front Of Player

So in my game there is a first person controller that falls inside 4 walls that lock it in(as if you were falling inside an empty skyscraper) and i want there to be cubes that generate at random x and z positions but staying inside the walls, before the player reaches them(so generating at a specific y aswell) i would like this to be constant and for the cubes to be destroyed after the controller has passed them.

i have kind of tried this already so but all i managed was a bunch of cubes generating every second in the same position, here is my code:

using UnityEngine; using System.Collections;

 public class Generator : MonoBehaviour {
 
         public GameObject cube;
         public int numberOfCubes;
         public int min, max;
         public float Time = 3f;
 
         // Use this for initialization
     void Start () {
                 InvokeRepeating ("PlaceCubes", Time, Time);
     }
     
     // Update is called once per frame
     void PlaceCubes () {
     
     
                 for (int i = 0; i < numberOfCubes; i++) {
 
                         Instantiate (cube,GeneratedPosition(),Quaternion.identity);
 
                 }
         
         }
         Vector3 GeneratedPosition()
         {
                 int x,y,z;
                 x = Random.Range(min,max);
                 y = Random.Range(min,max);
                 z = Random.Range(min,max);
                 return new Vector3(x,y,z);
         }
 
 }
 

i would also like to mention that i am a bit of a noob!

so if you could help me and tell me what i need to , be it creating a new script that would be great thanks!