I thought that was called a hammock sock???? I bought one from Dutch years ago and have used it a few times when the temperatures were extremely cold. I made sure to leave the screen opening near my head or not extend the cover all the way to the head end of my hammock and didn't have any problems. But it was for one night of camping next to my truck. It absolutely helped with keeping the wind out. Usually, the most difficult thing with hammocks is keeping underneath warm. With ground sleeping it seems to be the opposite problem. I always get cold on the top when on the ground.
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