I've been thinking about making a merino wool undercover/overcover. I'm not sure what it weighs per oz., but I could lanolinize merino wool and increase its waterproofing capabilities. I'm sure merino wool isn't cheap, but my field experiments with wool army blankets leads me to believe wool is a viable undercover/overcover against blowing rain and snow. It's not cuben fiber, but I'll bet merino wool beats cuben fiber on price. Ya gots to keep your down UQ/TQ dry!
Ripstop nylon might be lighter and more waterproof as overcover/undercover, but wool provides more insulative properties and more value in a survival situation. I could also multipurpose both undercover and overcover as daytime poncho.