What is the best way to prevent "hour glassing"? especially on mobile-home window frames I use round ribbed rubber spline(hollow) and foam spline but can't seem to get everything properly tight enough & still prevent it. I use rails with sliding frame holders on a 4x 8 table. use three kolders on all sides. one each corner & one in the middle. I've pre-bowed all the rails outward to try to get "pull-back" to normal.