Use an Adder RMK17-BP 2 piece blanking plate kit for when less than 3 ALIF modules are installed in an RMK17 rack frame ( all sold separately). In order to maintain suitable cooling airflow, if only two modules are installed within the rack mount, you are recommended to install an optional blanking plate.
Note: The outer edge of the blanking plate needs to overlap the ears of the rack mount itself, so you need to either install the blanking plate before the tray is installed or briefly remove the mounting bolts on the blanked side (while supporting the weight of the tray) to allow the blanking plate to be put in place.