That being said I primarily use M6 cage nuts since that's what APC gave us when we ordered hundreds of racks. Rack Solutions gives us a ton of 10-32 and 12-24 screws with each rail/shelf order so ...

Attach the plates to the pipe using the straps, then mount the monitor with screws. Screw the assembly into the back of your desk via the floor flange and M6 screws.