Add footer to posts based on category

Hi all,
I’ve searched around and found only a) plugins with too many options for me (like wp-insert) or b) hacks but I don’t want to do changes in the code in order to prevent problems with wordpress updates.

So I’m searching for a plugin which lets me add some text automatically to my posts depending on the category of the post (or the URL because the URL contains the category). The text (including images) should be displayed above or below the post. It should be possible of course to configure different footers for different categories, but each category only needs to have 1 footer. So for category A always footer A should be displayed, for category B footer B etc.
Thanks!