I've discovered a premade insulator for my hammock. It's not the warmest method, but it is probably the lightest and simplest. Go to Wal Mart or some other super center. In their automotive section, you will find the sun shades. Look for the kind that are metallic on one side and foam on the other. Here is an example. They should be less than five bucks.
To use, just stick it in your hammock, foil side up, and lay on top of it. The shiny side reflects the radiant heat back towards you, while the foam seems to be enough to protect against conduction (heat loss though contact). I wouldn't suggest using this method in sub-zero weather, but it could be used in connection with other hammock insulating methods to give you possibly an extra 10 degrees of warmth.
Here are some pictures of the setup.
This is the sunscreen folded up. Not very nicely, but I wanted it to be small.
This is it unrolled on the foam side. In front is a yardstick, so I'm guessing the entire thing is about four feet long or so. They also make larger ones. Normally there isn't a chunk taken out of the upper righthand side, but I was initially using this for a different project.
This is the sunscreen in the hammock, foil side up. Obviously.
As for durability, I wouldn't take a knife to it, but it has a fabric edging to it which mostly keeps it from tearing. Once a tear starts, however, it rips easily. But then again, for the price, you can just go buy another one.
I call it the Very Insulator because it's very cheap, very easy to make (or find in this case), and very easy to use.
Price: Under $5
Build time: None!
Availability of materials: Readily available
Durability: Relatively low
Functionality: Super quick setup
Portability: Weighs next to nothing, folds up pretty small