When I first started going lightweight several years ago, I was very skeptical about guidance which said that weight could be cut by foregoing stuff sacks and pack covers and instead opting for a simple interior liner such as a trash bag or shredder bag. Over time, I realized this guidance was reasonable and functional, although I still elected to use stuff sacks for certain things either purely for organizational reasons or to ensure absolute protection for my most important gear.
To me, my "most important gear" which must stay dry includes my down quilt. Simply put, in a pinch when everything goes wrong, I must be able to rely on it staying dry to possibly keep me alive. This alone was plenty enough to ensure I kept it in some kind of waterproof sack.
In the past, I used
cuben sacks offered by both Mountain Laurel Designs and
Zpacks. Both worked fine, but they were far from "absolute" protection because they had an open end usually for cinching shut with a drawcord. So if my pack were
unchacteristically submerged in water for example, and water somehow got past the
trash bag liner, there was still a chance that my quilt could get wet. At that time, no one was making a dry-bag style
cuben sack, so my options were to risk it or get a legitimate dry bag.
Fortunately, times have changed. Granite Gear (
http://www.granitegear.com/), likely the manufacturer with the most popular
thru-hiker packs on the Appalachian Trail, is currently manufacturing an "
Uberlight CTF3
DrySack" (
www.backcountry.com/outdoorgear/Granite-Gear-Uberlight-CTF3-DrySack/GRG0082M.html). This product is made of
CTF3 fabric which is indeed
cuben. It is half the weight of
silnylon while being nearly five times the tensile strength. One unique feature of the design is that it is sewn squared off which helps prevent dead air in your pack as it will fill space up more evenly. With a roll-top closure, it will really be a step up from a cinching-style sack. Depending on one of the four sizes, it weighs anywhere from .53oz to .74oz and you can find it from $38-$48.
Flatly - highly recommended.