By "dry sack" I assume you mean what I call "pack liner", a waterproof light thin liner bag that goes inside your pack. The term dry sack suggests to me a waterproof rucksack, a few do exist but are heavy.
Anyway another vote for Sea-to-Summit Ultrasil pack liners, very light and they do the job.
One thing I would say from experience is to get one that is oversize for the stated pack capacity, e.g, for my 48l pack I use a 60l liner. A 50l liner was difficult to close properly when the pack was full.