Example Drag & Drop area in a custom inspector for the Unity editor

TriggerContainerEditor.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 ();
             
                foreach (Object dragged_object in DragAndDrop.objectReferences) {
                    // Do On Drag Stuff here
                }
            }
            break;
        }
    }
}