Here's the finished product from a 20* REI down bag I converted to a top quilt. I bought the bag for $75 off geartrade.com. I haven't tested it yet but hope to soon.
The bags intial weight was just over 4lbs. I cut away the hood, zipper, and draft tubbing. I sewed a rolled hem on the edges where the zipper formerly resided. I finished the new top edge with grosgrain and added a shock cord with a cordlock.
The weight now is slightly over two pounds using a floor scale to estimate. I'll have an exact weight soon.
The only drawback I can see so far is that it appears the bag's original botton side had less loft than the top side. I tried to remedy this somewhat by adding down from the parts I cut away. I now have down all over my garage floor. That stuff is tough to handle.
Can't wait to try this new piece of DIY gear.