Help converting a character controller script to rigidbody

So I'm currently working on a 3D game in w$$anonymous$$ch 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 s$$anonymous$$ft 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;
             }
             w$$anonymous$$le (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);
     }
 }