Moving conveyorbelt

Hi, Noob here with a question about conveyorbelts. I searched and found a really nice solution (so far). I hacked the code a bit so I can activate/deactivate my conveyorbelt with a keypress;

 using UnityEngine;
 using System.Collections;
 
 public class ConveyorBelt : MonoBehaviour {
 
     public float speed = 2.0f;
     public float distanceTravelled = 0f;
 
     void FixedUpdate()
     {
 
         if (Input.GetKey(KeyCode.X)){
             Rigidbody rigidbody = GetComponent<Rigidbody> ();
             rigidbody.position -= transform.forward * speed * Time.deltaTime;
             rigidbody.MovePosition (rigidbody.position + transform.forward * speed * Time.deltaTime);
 
             distanceTravelled += speed * Time.deltaTime;
         }
     }
 
 }
 

now, I made a box, with a boxcollider and a (kinematic) rigidbody, attached the script and it works great when the conveyorbelt is at a fixed position in my scene, i place some rigidbodies on it and they move nice 'n smooth... but... i'd like to attach it to my vehicle, so i can slide those rigidbodies off it whenever I need to.... The problem is that the rigidbodies don't 'stick' to my conveyorbelt when i start moving around. I hope my description is clear, and if someone could give me a hint on how to approach this.

cheers! Mike