I have a WM Megalite regular. Though it's a 30 F. bag I've managed to sleep OK at 24 F. with all my clothes on.
Anyway I recently had it "overfilled" with one ounce of down (800 fill) for $38. including shipping. This will give me a 20 F. bag most likely. It's really lofty.
Maybe it's heavier than you'd like for a summer bag but I have wide shoulders and need the room. In addition it's big enough that I can wear insulated pants and jacket, like my Bauer down sweater if I need to go down to 10 or 15 F.
Also the Megalite opens fully to make a great quilt on hot to balmy nights.

