I've been thinking on this too, as I'm so broke I can't afford to pay attention. I think I've got my plan worked out, just need to get some materials.
I'm going to make a bottom half using the technique seen here,
I'll make it just longer than the hammock, and shaped right up to the top edges. Then fit a flat piece across the top with a zipper or lacing down the middle.
I'm thinking make the top so the insulation sags down on top of me.
I think I'll scavenge a yard sale cheapo bag or two to prototype, then maybe invest in some down and silk or similar for the finished item.