An implantable pulse generator includes a core assembly, a seal plug, and an outer layer overmolded over the core assembly adjacent the seal plug. The core assembly defines a core hole extending through the core assembly from a core interior to a core outer surface. The core hole has a hole outer portion and a hole inner portion. A first diameter of the hole outer portion is less than a second diameter of the hole inner portion. The seal plug is positioned in the core hole and has a plug outer portion aligned with the hole outer portion and a plug inner portion aligned with the hole inner portion. A third diameter of the plug outer portion is less than a fourth diameter of the plug inner portion. The outer layer leaves a top of the seal plug exposed.