WordPress Rewrite rule for extra query variable in category view

I have setup a custom post types called boxes that works fine and uses the default category taxonomy.

I have written a get_query_var at the top of my category.php to detect cp-type=boxes in the url and if so it only displays posts in that category from the custom post type boxes.

This all works fine but i want a pretty url

ideally something like /boxes/category/fancy/

i have tried using this method:

function my_rewrite_rules($rules)
{
    global $wp_rewrite;
    $my_rule = array(
        '^boxes/category/(.+)/' => '^category/' . $matches[1] . '?cp-type=boxes'
    );
    return array_merge($my_rule, $rules);
}
add_filter('page_rewrite_rules', 'my_rewrite_rules');

function my_query_vars($vars)
{
    $my_vars = array(
        'cp-type'
    );
    return array_merge($my_vars, $vars);
}
add_filter('query_vars', 'my_query_vars');

All this does is it throws a 404.