The Feathered Friends Sandpiper is a 2.1 lb semi-rectangular 850-fil down bag for one or if used with a separately sold zipped-in groundsheet serves as a quilt for two. It can be made in Epic which imparts serious water resistance so it can be safely used under a tarp. FF quality is second to none so it is a bag that you'll be happy with after DECADES of use. A bit more costly than what you're considering, but well worth it.

http://www.featheredfriends.com/Pica...ctangular.html

http://www.featheredfriends.com/Pica...undsheets.html