An implantable medical device having a case with therapeutic componentry contained with the case. A module has a rail around at least a portion of a perimeter of the module and is adapted to be mechanically secured to the case. The case has a rigid fastening channel adapted to receive the rail of the module. The rigid fastening channel has an opening allowing the rail of the module to drop into the rigid fastening channel through the opening and then slide along the rigid fastening channel to be mechanically secured to the case.