DragAndDrop EditorGUILayout.Box ?

Here, I use these everywhere.

 public static void DragDropArea<T>(string label, Action<T> onDrop, Action onMouseUp, float h = 25) where T : Object
 {
     HorizontalBlock(() =>
     {
         // draw a box area for our drag-drop
         GUILayout.Space(15f);
         Rect dropArea = GUILayoutUtility.GetRect(0, h);
         GUI.Box(dropArea, label);

         // cache the current event
         Event currentEvent = Event.current;

         // if our mouse isn't contained within that box area, exit out
         if (!dropArea.Contains(currentEvent.mousePosition)) return;

         if (onMouseUp != null)
             if (currentEvent.type == EventType.MouseUp)
                 onMouseUp();

         if (onDrop != null) {
             if (currentEvent.type == EventType.DragUpdated ||
                 currentEvent.type == EventType.DragPerform) {

                 // set the visual mode to copy
                 DragAndDrop.visualMode = DragAndDropVisualMode.Copy;

                 // if we dropped something
                 if (currentEvent.type == EventType.DragPerform) {

                     // register that this drag-drop event has been handled by this control
                     DragAndDrop.AcceptDrag();

                     // loop over the dropped items
                     foreach (var item in DragAndDrop.objectReferences) {
                         onDrop(item as T);
                     }
                 }
                 // since we've used the DragPerform event, we'll mark it as used
                 // (its type will change to EventType.Used)
                 // so that other controls ignore it
                 Event.current.Use();
             }
         }
     });
 }

Use these if you want manually register drag/drop for a custom field (Have a read at the DragAndDrop class to see what the methods do)

 /// <summary>
 /// Registers fieldRect for drop operations.
 /// sets the fieldValue to whatever that's been dropped, returns fieldValue in either cases.
 /// (could have used a ref parameter instead of returning the field)
 /// </summary>
 public static T RegisterFieldForDrop<T>(Rect fieldRect, T fieldValue, Func<Object[], Object> getDroppedObject) where T : Object
 {
     Event e = Event.current;
     EventType eType = e.type;
     if (fieldRect.Contains(e.mousePosition) && eType == EventType.DragUpdated || eType == EventType.DragPerform) {
         DragAndDrop.visualMode = DragAndDropVisualMode.Copy;
         if (eType == EventType.DragPerform) {
             DragAndDrop.AcceptDrag();
             fieldValue = getDroppedObject(DragAndDrop.objectReferences) as T;
             Event.current.Use();
         }
     }
     return fieldValue;
 }

 /// <summary>
 /// Registers fieldRect for drag operations. dragObject is what's being dragged out of that field.
 /// </summary>
 public static void RegisterFieldForDrag(Rect fieldRect, Object dragObject)
 {
     if (dragObject == null) return; // can't drag a null wtf!
     Event e = Event.current;
     if (fieldRect.Contains(e.mousePosition) && e.type == EventType.MouseDrag) {
         DragAndDrop.PrepareStartDrag();
         DragAndDrop.objectReferences = new[] { dragObject };
         DragAndDrop.StartDrag("drag");
         Event.current.Use();
     }
 }

 public static void HorizontalBlock(Action block)
 {
     GUILayout.BeginHorizontal();
     block();
     GUILayout.EndHorizontal();
 }

See this video of mine to see what I mean by "registering for a drag"