Example of simple component for keeping objects inside defined bounds by physically repelling them back when they exit.

ContainerGravity.cs

using UnityEngine;
using System.Collections;


[RequireComponent (typeof (BoxCollider))]
public class ContainerGravity : MonoBehaviour
{
	public float strength = 10.0f;
	
	
	void Start ()
	{
		if (!collider.isTrigger)
		{
			Debug.LogError ("ContainerGravity collider must be a trigger", this);
			enabled = false;
			return;
		}
	}
	
	
	void OnTriggerExit (Collider other)
	{
		StartCoroutine (BringBack (other));
	}
	
	
	IEnumerator BringBack (Collider other)
	{
		Bounds bounds;
		Rigidbody otherRigidbody = other.transform.root.GetComponentInChildren<Rigidbody> ();
		while (enabled)
		{
			bounds = collider.bounds;
			bounds.Encapsulate (other.bounds);
			
			if (bounds == collider.bounds)
			{
				yield break;
			}
			else
			{
				otherRigidbody.AddForce (
					(transform.position - other.transform.position).normalized * strength -
						otherRigidbody.velocity,
					ForceMode.VelocityChange
				);
				
				yield return null;
			}
		}
	}
}