How to add a class to meta boxes (to customize them in CSS)?

Answer given in your link is perfectly correct, you set it this way :

add_filter( 'postbox_classes_{post_type_name}_{metabox_id}', 'add_metabox_classes'  );

function add_metabox_classes($classes) {
  array_push($classes,'your_class');
  return $classes;
}