An implantable medical device (IMD) includes a housing that is configured to enclose internal components including at least a processor and a power source. The housing defines two major surfaces that are generally parallel to each other and one or more channels that are each configured to receive a lead and electrically couple the respective lead to the internal components, where each of the channels extend substantially straight in to the housing along an axis generally parallel to the two major surfaces. The housing may be configured to be mounted to a cranium of a patient such that at least one of the two major surfaces approximates a curvature of the cranium. The IMD may include one or more funneling walls that define a rounded and smooth transition from a sidewall of the housing to a surface that defines one or more mouths to the channels.