Draw Rect in Scene View from EditorWindow

Hi! I'm trying to create a basic level editor. The idea being a simple tile grid and when a user hovers their mouse over a tile in the scene view it will highlight.

I've got the grid to appear perfectly (see screenshot), however I can't figure out at all how I can draw a rect in the scene view. There doesn't seem to be a DrawRect in Handles and DrawRect in Gizmo's apparently cannot be used in OnSceneGUI.

 using UnityEngine;
 using System.Collections;
 using UnityEditor;
 
 public class LevelEditorWindow : EditorWindow {
 
     bool showGrid = true;
     Vector2 tiles = new Vector2(6,6);
 
     [MenuItem("Window/Level Editor")]
     static void ShowWindow ()
     {
         GetWindow(typeof(LevelEditorWindow), false, "Level Editor", true);
     }
     
     void OnFocus()
     {
         // Remove and re-add the sceneGUI delegate
         SceneView.onSceneGUIDelegate -= this.OnSceneGUI;
         SceneView.onSceneGUIDelegate += this.OnSceneGUI;
     }
     
     void OnDestroy()
     {
         SceneView.onSceneGUIDelegate -= this.OnSceneGUI;
     }
 
     private void OnGUI()
     {
         EditorGUILayout.LabelField("Level Editor");
 
         tiles = EditorGUILayout.Vector2Field("Tiles", tiles );
         showGrid = EditorGUILayout.Toggle("Show Grid", showGrid);
 
         EditorGUILayout.Space();
 
         if ( GUILayout.Button("Update Scene View") )
         {
             SceneView.lastActiveSceneView.Repaint();
         }
 
         EditorGUILayout.Space();
 
         EditorGUILayout.BeginHorizontal();
             GUILayout.Button("Load Level");
             GUILayout.Button("Save Level");
         EditorGUILayout.EndHorizontal();
 
     }
 
     void OnSceneGUI(SceneView sceneView)
     {
         float size_x = tiles.x;
         float size_y = tiles.y;
 
         if (showGrid)
         {
             // Draw all X grid lines
             for ( int i = 0; i <= size_x; i++ )
             {
                 // draw a line equal to x and the full height
                 Handles.DrawLine( new Vector3(i,0,0), new Vector3(i,size_y,0) );
             }
             
             // Draw all y grid lines
             for ( int i = 0; i <= size_y; i++ )
             {
                 // draw a line equal to x and the full height
                 Handles.DrawLine( new Vector3(0,i,0), new Vector3(size_x,i,0) );
             }
         }
 
         // check if mouse is over any tiles
         Vector3 mousePosition = HandleUtility.GUIPointToWorldRay(Event.current.mousePosition).origin;
 
         if ( (mousePosition.x > 0 && mousePosition.x < size_x) && (mousePosition.y > 0 && mousePosition.y < size_y) )
         {
             // mouse is within a tile
             int hoverTile_x = (int)Mathf.Floor( mousePosition.x );
             int hoverTile_y = (int)Mathf.Floor( mousePosition.y );
 
             // DRAW RECT OVER TILE HERE
 
         }
 
     }
 
 
 }