Slightly more elaborate example Drag & Drop area in a custom inspector for the Unity editor

TriggerContainerEditor2.cs

using UnityEngine;
using System.Collections;

using UnityEditor;

[CustomEditor (typeof(TriggerContainer))]
public class TriggerContainerEditor : Editor
{

    private SerializedObject obj;

    public void OnEnable ()
    {
        obj = new SerializedObject (target);
    }
 
    public override void OnInspectorGUI ()
    {
        DrawDefaultInspector ();
        EditorGUILayout.Space ();
        DropAreaGUI ();
    }
 
    public void DropAreaGUI ()
    {
        Event evt = Event.current;
        Rect drop_area = GUILayoutUtility.GetRect (0.0f, 50.0f, GUILayout.ExpandWidth (true));
        GUI.Box (drop_area, "Add Trigger");
     
        switch (evt.type) {
        case EventType.DragUpdated:
        case EventType.DragPerform:
            if (!drop_area.Contains (evt.mousePosition))
                return;
             
            DragAndDrop.visualMode = DragAndDropVisualMode.Copy;
         
            if (evt.type == EventType.DragPerform) {
                DragAndDrop.AcceptDrag ();
             
                // Do stuff with our newly dragged object
                foreach (Object dragged_object in DragAndDrop.objectReferences) {
                    AudioClip clip = dragged_object as AudioClip;
                    if (clip == null)
                        continue;
                     
                    Object trigger_prefab = AssetDatabase.LoadAssetAtPath ("Assets/prefabs/Trigger.prefab", typeof(UnityEngine.Object));
                    TriggerContainer t = obj.targetObject as TriggerContainer;
                    GameObject new_object = Instantiate (trigger_prefab) as GameObject;
                    new_object.transform.position = t.transform.position;
                    new_object.transform.parent = t.transform;
                 
                    AudioSource ac = new_object.GetComponent<AudioSource> ();
                    ac.clip = clip;
                }
            }
            break;
        }
    }
}