Unfortunately the described plugin is no longer available ("This extension has been unpublished for the following reason: UR5-Per Developer Request").
So i decided to write a small script to set the canonical tag fast and easy. It isn't a perfect solution but in think in most cases it does what it has to do
In i a few sentences: i use the article id to get the seo url. This works fine. If there is no article id, e.g. if a form from any extension is displayed, the script does nothing => no setting is better than a wrong setting
Download link and more infos can be found here:
Hope this helps!