Metasurfaces are engineered with specific shapes and sizes to interact with light in a unique way. By manipulating the design of the metasurface, it is possible to control optical properties of the surface such as its thermal emissivity. However, thin layers patterning techniques can lead to side-wall angles, thus modifying light propagation within the structure. Here, we introduce a one-mode model that fairly describes the propagation of light in structures with inclined sidewalls. We applied this method to two families of plasmonics resonators: nano Fabry-Perot and coupled nano Fabry-Perot with refractory materials ZrC and tungsten.