I understand, and I gave you an answer that is correct. You can’t accomplish what you are trying to do with just html or re_write rules; you need some way of determining whether of not the visitor is a “member” to decide whether to allow them to access the files or not.
If you don’t fell like “homebrewing” such an application, you might consider using the DH provided “one-click” installation of Joomla! which has the capability of restricting access to content to registered members only “built in” as one of it’s core components.
And, as I said, there are many such applications/systems out there - you just have to look and find one that you are capable of understanding, installing, and operating on your site. It’s either that, or write the scripts yourself - there really is no properly secure “shortcut”.
While there are kludges you could implement to “hide the link” from “non-members”, without the programming I described, once the link is “shared” by a member (or otherwise released “into the wild”), you won’t be able to prevent “non-members” from browsing to it.