Bivy sacks keep you warm while making a light weight waterproof covering for your sleeping bag.
Oh and this is in the stay warm contest so vote for me!
Step 1: Materials
~3 yards of Gortex
~3 yards of Black taffeta
Step 2: Step 1
pair up the fabrics with the white side of the gortex faceing out. Sew 1/2" away from the edges.
Step 3: Step 2
You could also use velcro but I hate it when things get stuck in it.