I'll second the bivy concept. But in reality, why do you want to waterproof it? If you use a propper shelter you should not need a waterproof bag. If you use a mini tarp then maybe the foot box, but even then, making your mini tarp larger would weigh less than the W/B fabric foot bivy. A little mist is not going to hurt your bag unless its realy cold. And if its realy cold, a better shelter is needed.
I'm not up to date on what materials are used, but they are not normal gortex, they breath better and somehow are just as waterproof (?).
Scott
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I had superhuman powers, but my therapist took them away.