Microwave energy can be advantageously used for materials processing as it provides high heating rates and homogeneous temperature field distribution. These features are partly due to the large microwave penetration depth into dielectric materials which is, at room temperature, a few centimeters in most dielectric materials. However, up to now, this technology is not widely spread for high-temperature material processing applications (>1200 °C), because its reproducibly and ability to sinter large size samples (>30 cm3) still needs to be improved. In this context, this paper describes both an empirically designed 915 MHz single-mode cavity made from SiC susceptors and refractory thermal insulation, and the 3D modeling of the process in order to improve our understanding of it. Different susceptors geometries and coupling slit position were numerically tested in order to better understand how these parameters impact the field homogeneity and the process stability. It was found that positioning the largest surface of the susceptors parallel to the electrical field allows a very uniform and hybrid heating of the material, while avoiding plasma or thermal instabilities. This was correlated to the 3D modeling results. Finally, thanks to a fully-automatized system this apparatus was used to sinter large size (~30 cm3) low-loss dielectric alumina samples. The sintered materials were subsequently characterized in terms of density, grain size distribution, and homogeneity. The reproducibility was also discussed, demonstrating the process efficiency and reliability.