Just thinking out-side the box.
What if you took 2 aluminized space blankets, and lapped them over, end to end and stitched together. You'd want it long enough not to crowd overall length of hammock. Then stitch up each short end leaving a open slot on each end enough to get end ropes thru. Then finish stitching the last 4" or so on each side.
 You'll have an "envelope" open at the top and enclosing the hammock.
It would be a wind/rain break, reflect heat back, even a spot for storage.
Wouldn't add much weight and you could probably stitch on home sewing machine? You'd have to play with the dimensions of hammock and slack needed, but it might give you the extra warmth you want?