Last summer I experimented with a light fleece blanket covered in a light nylon shell. I was inside a tent as I was with my daughter, which helped, and was wearing a light merino sweater. You don't save any weight but it saves you from carrying a bulky sleeping bag if you don't have a warm weather sleeping bag. You gotta check the climate extremes for that month though, and consider microclimates, like down on a beach on a clear night, or up on a hill clearing. I like the versatility and robustness of a wool or fleece blanket, but only mid summer. I think blankets are competitive down to 50F, quilts down to 40F, and sleeping bags generally take over below 30F. As a rule of thumb I think the shell weight should not exceed the insulation weight. Don't skimp on clothes when experimenting with bags.