Zpacks are just sewed across bottom and up one side, then taped.
It comes out kind of oval near bottom, and round in the middle.
Anything square or rectangular will come out round as well after stuffing.
Unless you use a hard shaped liner of some kind.
Unless you overcompress it and make it hard, it will take up whatever shape you cram it in to in the pack though.
When I need the room, I will put my down quilt into a zpacks medium drybag, squeeze it down , put my clothing in on top of it, squeeze it down, and roll it down.
It will fit crosswise in the bottom of my packs that way. Tighter fit in some, than others, but it fits at very bottom. Thats for big food carries, only time I need to squeeze it down that far.