Not an expert on flash but...
For a file to be non viewable via a browser the best way in my opinion is to place it outwith the document root.
so if your initial web page is within /home/username/htdocs you would create the file in /home/username or /home/username/flashdir etc..
This would however only be possible if flash uses the files server name and not a web orientated name.
Hope that this helps.