Rail mounting assembly of the type in which a top plate is secured to a frame by an elastomer body surrounding the top plate and bonded to both the top plate and the frame. Given the aforementioned structure, the plug can be replaced and selectively oriented with respect to the body. The instructions are extensions of the conventional JTAG operating structure. A parts support anvil is placed in a required position with respect to the rivet support anvil prior to formation of the second head on the opposite end of the rivet, to provide the desired amount of clearance. This arrangement makes it easy to secure a space for disposing the thermostat.