I made an underquilt today using a 2-person AMK HeatSheet and some polyester batting. I used double stick tape for tow sides and Velcro dots for the opening. I got a big roll of batting at Goodwill for $5 I'm using it in my SuperShelter under cover clone made from a silnylon poncho. I tried it this afternoon and it is warm-- I could feel it immediately. I'll be testing tonight. I do have concerns with condensation, but hopefully if it happens it will be below the main hammock fabric and it won't be a problem with the space blanket.
After getting it done, it dawned on me that the thing to use is an AMK Thermal Bivy. It is more expensive and heavier, but simple and more durable. I would add a few yarn ties through the bivy and batting with buttons for reinforcement. Rather than a fragile kludge job, this would be a very viable underquilt for use in any hammock with an accessory undercover and a 15 minute job to make.