First, the white sac that the parents are removing from the nest is a fecal sac, which is nature's way of keeping the nest tidy. The baby birds excrete their waste in this sac so that the parents can remove it cleanly. Second, you should do nothing to aid the birds in their efforts to raise and fledge their babies. Not only is it unnecessary, but it is also illegal to disturb wild birds in any way and may be harmful (the plastic cover could draw attention to the nest by predators). Be assured that the youngster do not need protected from the rain (the parents will do this) nor do they need a pillow to land on when they fledge. They will probably fly out of the nest and crash land many yards away in a shrub or in brush. This is normal. The parents will know exactly where they are, and will feed them. Please do not try to aid this process in any way. They should be fine, though in nature, there are always predators to be fed, and not all the youngster will survive. This, too, is nature's way, and it is normal.