Ed Speer hit on this lightweight way to ensure that you stay warm in your hammock many years ago. You could make this yourself in a lightweight nylon (eg .7 oz/yd2, such as this: http://www.tiergear.com.au/11/online-shop/argon-67) I don’t need the insulation for my knees as I always sleep on my back in a hammock, so mine would weigh half of the one shown in the photo ie less than a square yard of fabric plus two pieces of Evazote 18″ x 6”, so less than 2 ounces anyway.
Mind you, I find that a rectangular inflatable pad such as the Big Agnes or Neoair keep the hammock material far enough away from my arms when I sleep on my back so that I have no problem. The minimum I need is a couple of approx 6′ x18″ of blue foam pad which I just tuck into the sides of the hammock after I am settled. There are many more elaborate (and heavier) alternatives for the strong and well-endowed!