public function save_post($post_id)
 {
     $sidebar_id = $_POST['custom-sidebar-select'];
     if (isset($_POST['custom-sidebar'])) {
         $sidebar_id = Custom_Sidebars::get_sidebar_id($post_id);
     }
     update_post_meta($post_id, '_custom_sidebar', (bool) $_POST['custom-sidebar']);
     update_post_meta($post_id, '_custom_sidebar_id', sanitize_text_field($sidebar_id));
 }
 /**
  * Register a default sidebar to use when a post has no specific sidebar assigned
  */
 public static function register_default_sidebar($sidebar_id)
 {
     self::$default_sidebar_id = $sidebar_id;
 }
<label for="custom-sidebar"><?php 
_e('Register custom sidebar for this post?', 'custom-sidebars');
?>
</label>
<input type="checkbox" name="custom-sidebar" value="1" <?php 
checked(Custom_Sidebars_Details::has_custom_sidebar(), 1);
?>
 />
<label for="custom-sidebar-select"><?php 
_e('Use the following sidebar for this page:', 'custom-sidebars');
?>
</label>
<select name="custom-sidebar-select">
	<option value=""><?php 
_e('Default Sidebar', 'custom-sidebars');
?>
</option>
	<option value="none" <?php 
selected(Custom_Sidebars::get_sidebar(), 'none');
?>
><?php 
/* translators: as in 'No sidebar' */
_e('None', 'custom-sidebars');
?>
</option>
	<?php 
$metabox->sidebar_select_options();
?>
</select>