I finally got a sewing machine, so I chopped up my Lafuma X600 "45 degree" bag (that I got for $35 from REI) and had my lovely wife sew it up into a quilt. The bag weighed 20.5 oz; the quilt weighs 15. I didn't cut off any significant width, just the zippers and hood. The zipper was not quite full length, so I just left the existing footbox intact. I put 45 degrees in quotes up there because according to reviewers, it's more like a mid 50's bag; I haven't taken it below 65 yet, so I can't say for sure. But for the lower midwest, 50's is at least 2 seasons! At under a pound and 35 bucks, I'll take it.
The first pic is as a bag with the footbox inside out. The second is the outside of the finished quilt, and the third is the inside.
By the way, this doesn't change anything about my signature, since my wife did the dirty work