Instructions:

To stick cables smaller than the diameter of the cutouts in firmly, wrap electrical tape around the cable until it “clicks” in. 

To stack; only 2 of the holes are chamfered. Stack them in an alternating pattern so you screw through the chamfered hole into a non-chamfered hole to maximize amount of plastic available for the thread. Use 7/8" #8 wood screws for stacking, and whatever you want to mount it to the wall.