Thanks for the help.
Why not use enclosed in wall speakers? You get rid of the acoustic problems of on wall speakers and would easily fit behind your AT screen. You'll get better performance than a slim speaker as well. I wouldn't look at open back in wall speakers. They either need to have a back box or be in their own enclosure. The ones in their own enclosure are easier to fit if the drywall is already in place. Here is an article on baffled wall speakers (aka in wall).
For a thin speaker in your price range look at the Paradigm Millennia LP. There are many thin speakers but you are always giving up performance when you go too thin.