An excerpt from the World of Zero Physics Playground. A 2D controller to handle boxes accurately falling into precise holes.

FallingBox.cs

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(Rigidbody2D))]
public class FallingBox : MonoBehaviour {

    public float width = 1;
    private new Rigidbody2D rigidbody;
    private RaycastHit2D hit;
    public Transform origin;

    // Use this for initialization
    void Start () {
        rigidbody = GetComponent<Rigidbody2D>();
        width = 3.5f * 0.8f;
	}
	
	// Update is called once per frame
	void FixedUpdate () {
	    if (!Mathf.Approximately(rigidbody.velocity.x, 0))
	    {
	        float rayDirection = rigidbody.velocity.x > 0 ? -1 : 1;
	        Ray2D ray = new Ray2D(origin.position, new Vector2(rayDirection, -1));
	        hit = Physics2D.Raycast(ray.origin, ray.direction, width*2);
	        float halfWidth = width*0.5f;
	        Debug.Log(hit.distance);
            float collidedLength = origin.position.x - hit.point.x;
	        if (collidedLength > halfWidth)
	        {
	            Vector3 targetPosition = new Vector3(hit.point.x + halfWidth, transform.position.y, transform.position.z);
	            this.transform.position = targetPosition;
	            Debug.Log("Locking the box: " + targetPosition);
	        }
	    }
	}

    void OnDrawGizmos()
    {
        Gizmos.color = Color.red;
        Gizmos.DrawLine(origin.position, hit.point);
    }
}