I don't think the script supports sftp at the moment because I think it would require libssh2 being compiled into php for the php function ssh2_connect(); to work.
A work around could be:
You could set the ftp server to use optional TLS Encryption Support, and use strong 12-18 character passwords with Uppercase/Lowercase letters, numbers and special characters.
Strong passwords, ModSecurity, changing SSH port (22) and a good firewall (CSF) will stop 99.9999% of skript kiddies and hackers from getting in, especially if you set up the firewall level to high so it permanently blocks access attempts and port scans,