Gunnar, I have had three areas not dissimilar from the window bay behind your right speaker and they decorative insert closer to your left speaker. For two I had a trim carpenter create a two piece unit that I could “insert” or “stand up” inside the opening to create a temporary (only visible when in use) surface that I stored in a nearby closet when not in use. (You might be able to store them on the floor behind your couch?). In both cases I experimented with surfaces until I got it right. One ended up being 2” thick fabric wrapped RPG BAD panels mounted to the wood. Another ended up being 2” fabric wrapped RPG fiberglass panel with RPG Skyline diffusors mounted in front of that. You can get the fiberglass panels made to custom sizes. The Skylines are easy to cut to size and paint. Both worked pretty well. Finally, I also used two 4” thick RPG fabric wrapped panels that my trim carpenter added “feet” to that I could prop up in an opening. Those effectively ”killed” the hollow area even without sealing - four inches thick caught most of the wave and the bounce back from the rear of the cavity hit the rear of those 4” panels. Something like that is likely to work well in both of your (potential) problem areas.