A hermetic feedthrough for an implantable medical device includes an insulator body and a ferrule. The insulator body includes ceramic material and one or more electrically conductive conduits extending through the insulator body. The insulator is disposed in an opening of the ferrule. The insulator body includes a plurality of substantially flat surfaces that each include a plurality of edges. A rounded corner extends between adjacent edges of any two adjacent substantially flat surfaces. Each corner between any two adjacent substantially flat surfaces that face toward the ferrule has an average radius that is less than approximately 25% of a length of the corresponding edges.