Keep rigidbody rotation

I need to keep a rigidbody in the same position and rotation. When it's being pushed by another rigidbody, it should try to return to its initial pose, with a force proportional to the distance / rotation delta from its initial pose.

This code works very well for the position. The attached file is a short video showing the desired effect.
The rigidbody rotation is locked.

Code (CSharp):

1.  
2. using UnityEngine;
3.  
4. public class Keep_Physics_Pos : MonoBehaviour
5. {
6.     private Vector3 startPos;
7.     public float force = 50;
8.     Rigidbody rb;
9.  
10.     // Start is called before the first frame update
11.     void Start()
12.     {
13.         rb = GetComponent<Rigidbody>();
14.  
15.         if(rb == null)
16.             Destroy(this);
17.  
18.         Reset_Pos();
19.     }
20.  
21.     public void Reset_Pos()
22.     {
23.         startPos = transform.position;
24.     }
25.  
26.     // FixedUpdate is called once per physics step
27.     void FixedUpdate()
28.     {
29.         float dist = Vector3.Distance(startPos, transform.position);
30.  
31.         if(dist > 0.01f)
32.         {
33.             rb.velocity = (startPos - transform.position) * force * dist;
34.         }
35.     }
36. }
37.  

I can't figure out how to do the same for the rotation.

Thanks

 

I do want it to move - in a way. It needs to act kind of like a soft mattress, resisting to external forces lightly at first and then more strongly. The attached video shows the effect I'm looking for.

Using joints did not work, and since the position can be solved so easily, I wanted to find a similar solution for rotation.