The OD needs to be 1/4" a rod screw or piece of hose that is sealed at on end will work as a plug. I would just epoxy a small piece of hose and insert it in the
check valve. Or go to the hardware store to find a partialy threaded stud that is 1/4" OD and insert it to block it off.
I now see what your saying you did a boost leak test with the pump still connected. The reverse
check valve is for fluids I'm not sure what would happend if you pushed compressed air through it, It could possibly damage the
check valve and seal.
Just make sure you test the pump with the nozzle in place and make sure you get a proper spray pattern. The
check valve is only one way so if your pressure testing the system air could go through it without being stopped.