Unless the bottom of the bag has been baffled in such a way to be separate from the top of the bag, or you somehow manage to find a way to do that yourself, the top down will merely spread out and migrate to the bottom - you will be forever shaking it back into the top of the bag before use. Less trouble to buy or make a quilt, or a big agnes bag.
_________________________
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki

http://hikeandbackpack.com