Save meta box data from selected dropdown list in bbpress reply form

I add a show contents part referring to a workable previous post with link https://bbpress.org/forums/topic/add-custom-text-fields-to-reply-form/.
The below code can provide options for the user to selection. But the selection result could not be shown. I am not sure whether the selection result was added into metabox properly because I can not use the debug function. And another problem is that the “Field 1 ” was inserted into all previous reply posts.
Could anybody further provide help to solve this problem？

add_action( 'bbp_theme_before_reply_form_content', 'so_additional_content' );
function so_additional_content($post){
    $reply_id = bbp_get_reply_id();
    $meta_element_class = get_post_meta($reply_id, 'custom_element_grid_class', true);
    //$meta_element_class = get_post_meta($reply_id, 'custom_element_grid_class_meta_box', true); //true ensures you get just one value instead of an array
    ?>   
    echo '<label>Choose the size of the element :  </label>

    <select name="custom_element_grid_class" id="custom_element_grid_class">
      <option value="normal" <?php selected( $meta_element_class, 'normal' ); ?>>normal</option>
      <option value="square" <?php selected( $meta_element_class, 'square' ); ?>>square</option>
      <option value="wide" <?php selected( $meta_element_class, 'wide' ); ?>>wide</option>
      <option value="tall" <?php selected( $meta_element_class, 'tall' ); ?>>tall</option>
    </select>'
    <?php
}



add_action('bbp_new_reply_post_extras', 'so_save_metabox');

function so_save_metabox(){ 
    global $post;
    if(isset($_POST["custom_element_grid_class"])) {
         //check the capability: if ( !current_user_can( 'edit_post', $post->ID ))  return $post->ID;
        $meta_element_class = $_POST['custom_element_grid_class'];
        //END OF UPDATE

          update_post_meta($reply_id, 'custom_element_grid_class', $meta_element_class);
        //update_post_meta($reply_id, 'custom_element_grid_class_meta_box', $meta_element_class);
        //print_r($_POST);

    }
}


add_action('bbp_theme_before_reply_content', 'so_show_metabox');

function so_show_metabox() {
    $meta_element_class = get_post_meta($reply_id, 'custom_element_grid_class', true);
  //$meta_element_class = get_post_meta($reply_id, 'custom_element_grid_class_meta_box', true);
  // the problem is "Field 1" appear in every reply post
  echo "Field 1: ".$meta_element_class."End of testing "."<br>";  
}