I have been playing around with all kinds of combinations of pads and fillers to use with an under cover. I took a space blanket and crumpled it up and stuffed it in a 45 gallon LDPE (low density polyethylene) trash can liner, and sealed the top with a rubber band. I made up a couple and put them in the bottom of my under cover, and then put another pace blanket between the bags and my hammock. You can use the cheapest space blankets in the bags, but I recommend an AMK HeatSheet for the one under your hammock -- they are MUCH quieter and hold up better too
It works The bags are 1.4oz each and the space blankets are 2.5oz each, so the whole works is 7.8oz, plus my 9.7oz under cover. You get at least 2" loft, maybe a bit more. That should be fine for anything over 32F-- I would want to test it for the cold limit. It doesn't take long to assemble and packs small. I would just take the rubber bands off and roll it all up in my hammock for travel. This is warm, light, cheap, and very easy to make.
You can use the lightest, cheapest trash can liners you can find. I like the clear LDPE ones as I can see what is happening inside, but any color would work.
I want some really big ones now. They make big tubes for covering stuff like carpet rolls. If you could get some really wide (48" or so), lightweight stock, you could make one big pad with several blankets stuffed inside.
Update: I have re-invented the wheel, or the Garlington Insulator in this case. Ray Garlington designed an under cover and the stuffed bag insulation idea circa 2003.
I saw a reference to folding the space blanket inside the bag which I just tried and I like that. Folding gives better control over the coverage and I got four layers-- that is quite a thermal sandwich I think. I added the top space blanket, which gives more dead air space and perimeter coverage. I could see making four pads for colder weather, for better side coverage. I think using two pads and the space blanket will take care of my 3-season needs. I would be varying my topside insulation to suit the weather too.