As I understand, the reason down is such an effective insulator is that it traps small pockets of air and it is difficult for warmer or cooler air to circulate through because of that. As a result, if your down blanket were not contained in an airtight textile it would not provide much insulation. And if you did have an airtight textile, probably it wouldn’t matter if it was filled with down or anything else, the insulating effect would be more or less the same.
The down would be separated by vacuum, which is even better of an insulator than air pockets. It's just that vacuum flasks are harder to manufacturer than bags of fluff.
On the moon or in space you could get by with layers of reflective foil with a gap.
A friend once gave me a small sample of thermal insulation used for a satellite he worked on. It was several layers of metallised plastic (e.g. space blanket) with a very fine plastic mesh, almost like fine fishing net, in between those mylar layers to create small gaps between the metallised layers. I think it had about 10 layers like this.
