What worked out well:
Overall function of warming up the core, without overhearing the back was achieved. Apron-vest is easy to put on. It protects from the wind, it doesn't interact with backpack shoulder straps, thus allowing air to escape through armscyes and prevent overheating in the chest area. It kept me warmer and happier at a very low price in weight and space occupied in my bag. It also works well in combination with rain coat, where coat protects from water and apron-vest under it adds a warmth, so walking under the rain is much less miserable.
To be continued. There are ideas of separate down-sleeves lurking in my mind.
Features to think more about:
There is a mechano-magnetic buckle there, as well as at the waist, it snaps close without precise positioning (it's a FidLock magic). Even though stiffness of a buckle on the neck doesn't bother too much, because it is worn over other clothes, I still feel that there should be kind of cushioning that will go under buckle and will totally prevent contact with the skin. It is tricky, cushioning have to have a structure to stay under buckle and not to bunch up or slide sideways from under.
Another type of fastener could solve a problem too. It could be a magnetic one as well, because it's very convenient to operate blindly, but with a soft body, maybe a row of small magnets embedded in the collar ends.
Projected length is good, but I missed that due to the loft of the down it shortens in the direction perpendicular to down compartments, most of the time this direction is length. I should create an equation that will allow to calculate by how much pattern need to be lengthen in order to compensate for down puffing up the surface and therefore shortening overall length. Though probably I will just approximate that length should be increased by 7 or 10%, it will do most of the time, though I understand that it depends on the height of the compartments, as amount of curvature should be somewhat different for 6" and 2" chambers.
Fidlock buckle works perfectly at the waist, the issue is with the pieces that connect straps and main piece.
The pieces don't have down inside and because fabric that is used is very light and packable it doesn't retain shape. Problem appears at the moment of donning, because that triangular connective pieces are pretty twisted, when taken out of a bag, so they require flattening before strap could be closed, otherwise chances are high that one of the buckle pieces would be upside down and will not work.
Solution is simple - adding some interfacing to that triangles so they retain shape.