How do I secure a non-OS FTP server?

SFTP, basically SSH tunnels. It's a trivial task with Linux or Mac systems and doable under Windows, just a little bit of a pain. SFTP uses encryption and it has error checking unlike FTP. You will need to open up port 22. You can install free SSH/SFTP support. You'll need to byp the router most likely and do it point to point. Setting up essentially a VPN but without the headaches and hles of a full scale VPN.