After a bitter night a couple of weeks ago in my 2 season bag..... I just bought one of these http://www.surplusandadventure.com/...litary-issue-extreme-cold-weather-523330.html but on unwrapping it the outer shell is just cotton and looks like it will soak up like a sponge and get filthy real quick? Is there a light weight cover or something, I mean you wouldn't want a bivi bag if you was in a tent.