Make Author Archive Page URL be a Subdirectory of a Custom Post Type URL

Change custom-post-type in the following to the name of your CPT, both the slug and post_type= in the query vars. Visit your Permalinks Settings page to flush rewrite rules. You could also put this in a plugin and flush rules on plugin activation.

function wpa83047_author_rewrite_rule(){
    add_rewrite_rule(
        '^custom-post-type/author/([^/]+)/?$',
        'index.php?author_name=$matches[1]&post_type=custom-post-type',
        'top'
    );
}
add_action( 'init', 'wpa83047_author_rewrite_rule' );