Limiting Player Movement in 2D game?

Hello y'all!

i'm making a small game to use in a sound design portfolio. the game is 600 x 900 for web and all takes place within 4 game object quads i've placed around the game as boundaries.

I'm using a script i found here: http://pixelnest.io/tutorials/2d-game-unity/player-and-enemies/ for the character controller which works great because it allows the player to float around freely and interact with some circle objects that are also floating around. My issue is though, that with the player controller script as is, the player can fly over and endlessly past the borders!

I really don't know where to start as far as editing code to limit a character's movement so any help on making the character limited to those borders would be of great help.

Here's the script:

 using UnityEngine;
   
 /// <summary>
 /// Player controller and behavior
 /// </summary>
 public class PlayerScript : MonoBehaviour
 {
   /// <summary>
   /// 1 - The speed of the ship
   /// </summary>
   public Vector2 speed = new Vector2(50, 50);
   
   // 2 - Store the movement
   private Vector2 movement;
   
   void Update()
   {
     // 3 - Retrieve axis information
     float inputX = Input.GetAxis("Horizontal");
     float inputY = Input.GetAxis("Vertical");
   
     // 4 - Movement per direction
     movement = new Vector2(
       speed.x * inputX,
       speed.y * inputY);
   
   }
   
   void FixedUpdate()
   {
     // 5 - Move the game object
     rigidbody2D.velocity = movement;
   }
 }