function gu_top_hero( $meta_boxes ) {
  $prefix = 'gu_field_top_hero_';

  $meta_boxes[] = array(
    'id'         => 'gu_top_hero',
    'title'      => 'Top Hero Block',
    'post_types' => array( 'page' ),
    'context'    => 'normal',
    'priority'   => 'high',

    'fields' => array(
      // Image field
      array(
        'name'   => 'Top Hero Image',
        'id'     => $prefix . 'image',
        'type'   => 'image_advanced'
      )
    )
  );

  return $meta_boxes;
}