+1
Thin plastic can reflect highs while still letting bass go through and get absorbed by underlying layers. You’ll need to get it distributed on opposite walls so the sound can bounce around again. By selectively livening up parts of the room you may be able to achieve a desirable effect where immediate early reflections are absorbed but later reflections can stay alive for a while and reach your ears to create some spacious ambience. If you can prevent early reflections from reaching your ears for the first 10 to 15 milliseconds and then have some ambience sustain after that in the room, that should sound really good. That corresponds to a path length of about 14 to 20 feet, so the sound from the first reflection should travel an additional 14 feet to your ear after the direct sound from the speaker arrives. Delayed reflections that cross your head at angles of 60 degrees or more are desirable to create maximum inter-aural difference. Reflections from straight ahead and behind are less useful and should be deflected to the sides of the room.