Take a large lawn and leaf bag, make two slits up one side from the opening about 2/3 of the way to the “bottom.” Make one more slit up the middle of the same side, about 1/3 of the way. If you slip this over your pack, positioning the long slits so they coincide with the straps, you can then tie the strips you just made to the large flap, low under your arms, and you have a very lightweight and cheap rain cover for your pack. We even carry an extra in case one gets ripped up on branches, and still are carrying less weight than those heavy nylon covers you can buy. If this doesn’t make sense and anyone wants more info, I’ll take a picture.
.