I use a Montbell spiral down bag that stays loose in a light netting sack at all times except when in the pack and on my back. When I used bags with synthetic fills, I found them much inferior with respect to warmth for weight, not to mention comfort in widely varying temps.
Made a bag with overlapping shingle construction (like that used by Northface) using Thinsulate Lite Loft that was the best of the synthetics tried. Note, this was not like the Thinsulate made for gloves, etc.; but rather, a puffy batt. The shingle construction is only sewn crosswise, giving the bag an appearance of a down bag, so allows most of the Lite Loft to decompress except at the top and bottom edges of the batt where sewn.
However, the Montbell could be carried outside in a larger waterproof sack on the pack’s top shelf were it necessary; but have not found this necessary for any reason, so it goes more compactly inside the pack when hiking. So the Lite Loft and some other synthetic bags were sent to the Ukraine at their request.
Not sure this is helpful because am not sure why your bag must be tightly stuffed at all times.
