Should I attach this to a folder as a folder action or something like that
Folder Actions won't help here. There is no Folder Action that is fired when you logout. Therefore some kind of logout script is the way to go.
but what if I wanted the user to be who ever is logged in and not "ben"?
That's what the 'path to' command is for. It will return the current location of common directories (desktop, preferences, library, etc., etc.) based on the current user and language settings (the desktop is not called 'Desktop' when running a French OS, for example).
<pre class=command>set theFolder to (path to Desktop as text) & "Move to Trash:"</pre>