I took the back out of a 20 degree Zirkel BA. It had some down on the bottom side of the zipper. I took the zipper out and move that portion to the top. I left a couple of feet of the bottom to form a footbox. I also cut off the hood. If memory serves me correctly, I removed about 3 or 4 oz of material.
It works well in my DIY bridge hammock.
I also have a lighter weight BA that is going to get a similar treatment.