Interesting thought to follow. My fulltimer has a 1/2" of poly batting on a sewn on undercover but is mostly to block the airflow from the fan which runs 24/7. Nighttime temps inside run in the 50/60's winter and I'm more than comfortable. The little ones may need more or less. The current sewn on undercover does tend to load the edges of my hammock pulling them down.
The present version of my hiking hammock has poly batting on an undercover seperate from the hammock, hammock and undercover only attached at the gathered ends, and works adequetly so far even if it is a little bulky. So I'm thinking maybe a light, possibly polyester suit lining material with an attached fleece layer as a seperate undercover to the hammock. Might have give this a try myself with my tablecloth hammock. Hopefully my ramblings make sense.
Most of us end up poorer here but richer for being here. Olddog, Fulltime hammocker, 365 nights a year.