I have a theory or two about keeping warm in a hammock in cold weather.
1) stop air flow under you - I try and achieve this by diagonal tarp and bringing the corners each side down low.
2) trap the warm air (warmed by your body) under you - like a skirt does. I keep meaning to test this out. In principle just a light weight wind proof sheet hung over the hammock edges should achieve this. Even better if it was a tube, so the warm air couldn't easily be blown away. The physics is quite different to insulation over you - where the heat is trying to rise into the sky above you.
1) stop air flow under you - I try and achieve this by diagonal tarp and bringing the corners each side down low.
2) trap the warm air (warmed by your body) under you - like a skirt does. I keep meaning to test this out. In principle just a light weight wind proof sheet hung over the hammock edges should achieve this. Even better if it was a tube, so the warm air couldn't easily be blown away. The physics is quite different to insulation over you - where the heat is trying to rise into the sky above you.