mhansen - 5:46 pm on Dec 17, 2012 (gmt 0)
If all your efforts can be done away with by one person sharing the link in a forum somewhere, it's going to be harder to manage.
Have you considered adding a plugin, or using a third party service to manage the download? How about your auto-responder, do they offer that option? MailChimp and others can handle an encrypted link within their own servers.