glen's plans look goood. You may also consider shifting your speakers to the right facing the corner where you now have the TV hung. An absorber could be run accross that corner easily. Also, if leaving that desk around the wall you might consider running 12" batt under it where it meets the wall to keep it from resonating. And then making another soffit mounted absorber run around the ceiling of the front wall either with panels or 12" batt wrapped in fabric. You could use wire stringers to place it and then wrap it with the fabric.
As with glens idea , the rear wall seems close and would need thicker panels than what glen has drawn to be effective. The side corners of your trapazoid will need treatment as well. And oabove you will require a panel if not already treated.
Regards,
Brett