Lay the Windstop on the floor upside down and move the metal sliders all the way out to the 'latched' position. Drill a small hole in the center of the slider beside the plastic bracket that holds the slider down.
Now, take the four screws out of the plastic cap and remove the slider. Using a larger drill than you used to drill the hole, clean up the metal burrs that were left from the initial drilling.
Now replace the slider and the plastic cap. Repeat procedure for the other slider. Your local hardware store should sell small locks that will fit the holes that you have just drilled. I got two locks that were sold as a set that are keyed alike.