Here is an option: Sea to Summit Reactor

However, you may want more warmth than this will provide. If your going to be living out of your backpack, I'd search for a deal on a new -15 degree bag (from a good manufacturer). I have had good luck with north face (but add 5-10 degrees to there rating) mountain hardware (rating is right on!) and REI (warmer than rated by 5 degrees or so). Before I invested in my mountain hardware system, I had a 15 degree bag and a 32 degree bag that fit over my 15 degree... that coupled with a goot pad and a bivy sack kept me warm to -20 in the open and -45 in snow. It was a good system that gave me options for all conditions I encountered.

I have one of these for emergencies, and lent it to a member of my rescue squad durning a drill. She had a cheaper 30 degree bag that wasn't cutting it for the 25 degree temp, we placed the Thermo-Lite bivy over her bag and she was comfortable for the night.