The drawback though is that doing this one or twice is not so cumbersome, but if your downloading quite a few models this is going to get really old, really fast.

 If a session-id is provided the user has to identify once (entering the code). Then he may download as many as he can. If this isn't enough, identification may expire after some downloads. Similar as with a password protected site. Same concept as with "serious" porno pay sites. This guys know how something like this works :)