Help converting a character controller script to rigidbody

So I'm currently working on a 3D game in which the main character is a cube. The issue is that the script I've been trying to make work for my movement uses the character controller, and the character controller capsule collider is causing quite a few problems. My idea was that whatever way the camera is facing is forwards for the character. I've been trying to shift the script from utilizing the character controller to utilizing rigidbodies so I can use a cube collider, but I can't get it to work. if anyone had any advice or tips, that would be great. Thanks! Here's the script with all the character controller pieces in it:

 using UnityEngine;
 using System.Collections;

 public class CharacterControls : MonoBehaviour  {

     [SerializeField]
     private CharacterController controller;

     public float deadzone = 0.25f;
     public float turnSpeed = 800.0f;
     public float jumpForce = 3.0f;
     public float gravity = 10.0f;
     public float speed = 8.0f;
     public bool jump = false;

     private Vector3 moveDir = Vector3.zero;
     private float lockRotation = 0.0f;

     void Start ()
     {
         controller = GetComponent<CharacterController>();
     }

     void Update() 
     {
         // Is the controller on the ground?
         if (controller.isGrounded) 
         {
                 jump = false;
             // Assigning Translation Controls
             moveDir = new Vector3(Input.GetAxis("Horizontal") , 0, Input.GetAxis("Vertical"));
             // Align forward vector of movement and camera. Turn it off when a camera isn't assigned. 
             moveDir = Camera.main.transform.rotation * moveDir;
             // Rotation parameters based on stick direction
             if(moveDir.sqrMagnitude > deadzone) 
             { transform.rotation = Quaternion.LookRotation(moveDir); }
             //Jump
             if (Input.GetButtonDown("Jump"))
             {
                 moveDir.y = jumpForce;
                 jump = true;
             }
             while (jump == false);
         }
         // Applying gravity variable to the character controller when jumping
         moveDir.y -= gravity * Time.deltaTime;
         // Applying move direction to the character controller
         controller.Move(moveDir * speed * Time.deltaTime);
     }
     void LateUpdate()
     {
         // Lock rotations x and z to stop tilting. 
         controller.transform.rotation = Quaternion.Euler(lockRotation, transform.rotation.eulerAngles.y, lockRotation);
     }
 }