Example of the three masks employed to estimate the diffuse γ-ray emission in ROI1. Upper panel: mask 𝒜 used to exclude the Local Spiral Arm and a large fraction of the inner Galactic Plane where most of the γ-ray sources are expected to be located. Central panel: mask ℬ used to remove the emission from the most luminous clusters that should have been detected by LHAASO (this example mask is the one built based for the specific source realization shown in Figure 2 and assuming the Kraichnan diffusion coefficient). Lower panel: mask 𝒜 ∪ ℬ. For comparison, in all panels, the M1_L mask is shown as black contours.