My son has done some hammock camping, and has done some sections of the Appalachian Trail AT. He carries tyvec for a tarp. One night, he read or saw a youtube vid on using tyvec, and a emergency mylar blanket as a compact under-quilt, and went to work putting it together. By time he was done, it was a windy 17F outside, and he had to test it out.
He was pretty impressed with how much warmer he was than with his bag alone. He stayed out for a few hours, got some sleep, but finished the night inside in our heated cave
I got a little sticker shock when I saw the prices for under-quilts, so I decided to make one for him for Christmas.
I picked up a rip-stop nylon hammock from Kmart, bungee cord from Ramsey Outdoor, and polyester insulation from Walmart. It weighs 2 pounds and is 45 inches wide, by 54 inches long. So I guess it is a three quarter's under-quilt. I had intended to get thinsulate primaloft, but I was running out of time. I doubled up the insulation in the center, and not so much on the outer edges. I figured that if he continues to use his bag, that the insulation in his bag will be compressed in the center of the hammock, more so than along the edges, and the top, and therefore it would be better for the under-quilt to insulate the bottom, center more.
I believe that the primaloft would have to be quilted, whereas the batted polyester didn't have to be. I did tack it here and there to make sure it didn't move around too much though. If he likes it, but wants more compressibility, we can always open it up and use primaloft.