Light weight, but only somewhat effective
The trade-off between light-weight gear and adequate performance is perhaps best exemplified in this pack cover. It will get damp. It will form a puddle inside if the pack is left out in the rain for more than an hour. But it sheds enough rain that your packcloth will only get damp but not soaked. I took four backpacking trips of a week each with this cover and each time the material soaked through in about half an hour of rain. I returned the cover because it did not fit my needs, since my ten year old cover both fits better and is more waterproof than this cloth. It might fit your need, though, so don't ignore the tradeoff of dampness versus weight savings. Note that my Osprey Exos 58 (about 3500 cu inches) was far too small for the medium cover, which was sized for an expedition pack. If you go light, get a small size.