Using the single-mode relaxation-time approximation, the macroscopic thermal conductivity 𝜅 latt is calculated as a sum of modal contributions evaluated on a regular grid of wavevectors sampling the phonon Brillouin zone (Eq. 2 in the text). Appropriate sampling meshes for each of the models investigated in this work were determined by explicit convergence testing to be 9 × 9 × 9 for the pristine and filled models based on the CoSb3 conventional cell and 11 × 11 × 11 for the pristine model based on the primitive cell.