Surprised it has not been mentioned, but at one point, there were down pants that could be zipped at the legs to make an "elephants foot" half bag, significantly reducing the surface area problem, and a down jacket that zipped to the pants to prevent drafts. The "elephants foot" half bags are still available, I know, but I haven't seen the pants for awhile. If one can sew and handle down, one could make a pair.

I remember being quite interested in these at one point, but they were way out of my price range. The jacket and pants together also weighed slightly more than the sleeping bag I was using.

The system had a French name (translated as Elephant's Foot) and seemed aimed at the climbing community.

Since this approach seems to have largely disappeared, I suspect it was not fully successful, but do not know what the drawbacks were.

best, jcp