At least one surface of an implantable medical device is concave along at least one axis such that it substantially conforms to a surface within a patient, such as the cranium, when it is implanted on that surface. In some embodiments, the surface of the implantable medical device substantially conforms to an arc with a radius that is between 4.5 and 9.5 centimeters, and is preferably approximately equal to 7 centimeters. In some embodiments, the implantable medical device comprises a plurality of interconnected modules, and an overmold that at least partially encapsulates each of the modules. In such embodiments, at least one surface of the overmold is concave along at least one axis. Further, each of the modules of such an implantable medical device may comprise a housing, and at least one surface of at least one of the housings may be concave along at least one axis.