If you can get by with a 15 degree bag. I just picked up a Marmot Aspen 15 degree bag at Dicks. 600 FP goose down weighing 2lbs 8oz. Made exclusively for Dicks but Marmot stands behind this bag.
I may have missed this, but WHERE in the desert are you going that you'll need a 0 degree bag?
During winter, I almost LIVE in the local high-desert, and we never see 0-degrees...
That, and a good thermarest and tent make it much warmer.
That said, I have a US Army issue arctic bag (-20) I'll sell... It's 7lbs, comfy, and only used once despite it's 50+ years old...
Quilts work as well as bags, less weight at any given temp rating comparision too...
JRB Old Rag Mtn easily into single digits, possibly zero for some...on sale about $ 250...packs to 8x8x10...25 oz
Ounces to Grams.
www.jacksrbetter.com ... Largest supplier of camping quilts and under quilts...Home of the Original Nest Under Quilt, and Bear Mountain Bridge Hammock. 800 595 0413