Premium Acoustic Slat Wall Panel 2.4m x 0.6m Grey | DIY at B&Q
12 Pcs Sound Proofing Panels, JAMELO Hexagon Self-adhesive Acoustic Panels with Beveled Edge 14" X 12" X 0.4" High Density Sound Absorbing Panels for Recording Studio & Home & Offices Black :
JJM 130
I built acoustic panels (haven't hung them yet :)) following a tutorial on youtube. Total cost ~$30 : r/BudgetAudiophile
6pcs Acoustic Panel Sound Absorption Foam for Home Theater Studio Gray