What I did for a summer bag was go to a army-navy surplus store and get a poncho liner. Then I installed a kit that would turn it into a sleeping bag: I borrowed someone's sewing machine to attach the zipper, and then threaded a cord through the head area so it could be drawn shut. It was easy, and it packs down small. Not too spendy either. It's not super warm, but it works fine if I wear most of my clothes. I've had it down to about 40 degrees comfortably, though I was in a bivy, fully clothed, and also had a full-length sleeping pad.