Use statically.io CDN with WP Offload Media

add_filter('as3cf_get_attachment_url', function($url) {
    // Regex matches both http and https.
    $url = 'https://cdn.statically.io/img/' . preg_replace('#^https?://#', '', $url);
    return $url;
});

More hooks of WP Offload Media can be found on amazon-s3-and-cloudfront-tweaks.php