I recall seeing a guy use a "blizzard bag" I think it's called, as a pod.
It has one major advantage over blankets: baffles with reflective layers on top & bottom. The baffles are elastic so they hug the user, and the air space serves as insulation.
With your idea, I believe getting and maintaining a uniform airspace will be VERY difficult, and without it you will have outside moving air currents rob you of heat.
Good thinking, don't give up on creativity, report back on how it goes, and don't give up on a good underquilt!
-- Funny Money
Love 'em while you got 'em