I'm just now experiementing with my own cold weather cover. Hennessy sells one that you can stuff with dead leaves. Mine uses vapour barrier plastic and I use baffles to maintain some dead air under the hammock. This also seems to work great for windy days too when there is a lot of heat loss from under the hammock.

I also use a silver windscreen reflector to lay on and wear only fleece. You don't really need a thermarest in a hammock.