I must disagree Eileen. I have actually separated and timed my molly. She drops fry at exactly 6 week intervals, not 4 weeks. I find that covering the filter inlet with any netting, a bag or panty hose or whatever, means a real problem trying to keep the cover from blocking all flow. Instead I use a piece of filter sponge with a slit in one end and slide it up over the filter inlet so that it looks like my picture. It takes forever to become blocked and reduces the flow rate at the surface of the sponge because there is such a large surface area for the flow to go through.
If I really want to save a lot of fry, I move the female to a well planted birth tank that is big enough for her to live in, not a breeder box. The tank needs adequate cover. That way she can relax and have her fry when the time comes.