I’m not with cesium, but I cloned the material from their plugin folder and managed to make a modification. I ended up clonging GltfMaterialWIthOverlays, RasterOverlay and GlftMaterial making my own version. Once I did that I could overlay it onto the material from the Terrain and it took my modifications. I haven’t done extensive modifications to it yet, but was able to mask the globe down into a small square using a box mask.