Some means to join parka and sleeping bag in one already exists and is known as “elefant leg”. But this solution suffer from tightness, as volume of resulting sleeping bag is the same as parka’s. More volume needed, and it is possible with frontal insert, look at the sketch.
Frontal zipper on parka embed the frontal flap on the “tail” from left and right sides (look A and B). And additional bottom zipper on parka connects with its vis-a-vis around the tail (look C). There is a little detail not shown on sketch, a kind of face cover on top of the flap.
What is the idea?
To join two things in one.
What is the difference?
More inner space.
What is the advantage?
Less overall weight of two things.
What are the disadvantages?
Redundant weight of standalone parka.
Moisture trapped in parka.