If it was me, I would get rid of the sag in the lower rails by changing the linear bearings from 360' ones to 270 deg ones ( ie ones that leave one segment of the bearing open ), and put the opening in the bearing, at the bottom ( best for sagging) or outside. you can then structurally support the rod/s either from underneath ( preferred) or one side. here is a photo that shows a open pillow bloch