Seems like it would be light and small for 40dF warmth.

http://www.backpacking.net/makegear/falk-sleepbag/index.html

I was thinking about trying something different and puting a fleece liner inside of one of these instead of above.

Its a bit heavier at 7 oz but pretty tough material.

http://www.adventuremedicalkits.com/product.php?product=146&catname=Essentials&prodname=Thermo-Lite®%202.0%20Bivvy


and

http://www.backpacking.net/makegear/liner-quilt/index.html