DIY Acoustic Panels
There are kits that you can purchase to build customizable acoustic panels. One such site I found: https://www.acoustimac.com/acoustic-insulation-materials/diy-kits/acoustimac-do-it-yourself-421
They even have Bass-trap kits that you can build 🙂
I believe @123toid HT build thread has a post about finding the perfect spots to place these panels to absorb the sound.