Fika-Tarkov/mods/Borkel's Realistic Night Vision Goggles/BepInEx/plugins/BorkelRNVG

Place any textures you want to use as masks in MaskTextures, just make sure they are PNGs and have the name of the NVG mask you want to replace.
If you want to change the area where the NVG effect is applied, you have to change the textures in LensTextures
Each LensTexture has to be the same size as its corresponding MaskTexture