How to use the cpw_excerpt filter to add additional excerpt field to be used by the category posts widget

cpw_filter.php

<?php
/*
Plugin Name: Unique excerpt for Category Posts Widget
Plugin URI: http://mkrdip.me/category-posts-widget
Description: Adds a meta box for all posts in which the user can edit the excerpt that will be displayed at the category posts widget
*/

/**
 * Register meta box(es).
 */
function cpw_excerp_register_meta_boxes() {
    add_meta_box( 'cpw_excerpt_meta_box', 'Excerpt for Category Posts Widget', 'cpw_excerpt_metabox', 'post' );
}
add_action( 'add_meta_boxes', 'cpw_excerp_register_meta_boxes' );
 
/**
 * Meta box display callback.
 *
 * @param WP_Post $post Current post object.
 */
function cpw_excerpt_metabox( $post ) {
    $content = get_post_meta($post->ID, 'cpw_excerpt',true);
    // seems like wpautop needs to be set to make the editor wrap text in a <p>
    // element as the core excerpt does
    wp_editor($content,'cpw_excerpt',array( 'media_buttons' => false, 'wpautop' => false ));
}
 
/**
 * Save meta box content.
 *
 * @param int $post_id Post ID
 */
function cpw_excerpt_save_meta_box( $post_id ) {

  if ( wp_is_post_revision( $post_id ) || wp_is_post_autosave( $post_id ))
		return;

  if (isset($_POST['cpw_excerpt']))
    update_post_meta($post_id,'cpw_excerpt',$_POST['cpw_excerpt']);
}

add_action( 'save_post', 'cpw_excerpt_save_meta_box' );

/**
 * Do the actual excerpt filtering. If we have an explicit excerpt set, use it,  otherwise use whatever is already calculated
 *
 * @param string $excerpt The current excerpt
 * @param WP_Widget $widget the widget object from which it is possible to get the information of its type an dID number
 * @param int $length the configured request length for an excerpt. A zero is passed if no value is actually configured
 * 
 * @return The excerpt configured for the current post if exists, otherwise the current except
 */
function cpw_excerpt_filter($excerpt,$widget,$length) {
    $t = get_post_meta(get_the_ID(),'cpw_excerpt',true);
    if (!empty($t))
      $excerpt = $t;
    return $excerpt;
}


add_filter('cpw_excerpt','cpw_excerpt_filter',10,3);