Mandatory (meta) fields in post edit screen

With use of custom post types exploding, and WordPress use beyond ‘pages’ and ‘blogging’ being more clearly defined and easier to implement, creating ‘things’ that have compulsory data elements seems a common requirement.

There are quite a few support posts about required or mandatory fields that appear unsatisfactorily answered stretching back over a few years.

How does one make a field compulsory? Is it possible to hook into save_post, check for the field and return a message?

Here is how I thought it might be done:

add_action('save_post', 'my_check_mandatory');
function  my_check_mandatory($post_id){
  $post = get_post($post_id);
  if( $post->post_type=='my_post_type' && empty($_POST['mandatory_field']) ){
    return new WP_Error('missing_field', __('Mandatory (Exhibitor) field missing'));
  }
}

Any takers on how to actually do this?