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!

I would like a little bit more details. How quickly do you want the cubes to spawn.

A small tip
The cubes you want should be prefabs with a destroyer script set to them.
The script should have OnTriggerEnter used.
Then destroy the gameobject.