Anyone use one? I have a good sleeping bag, but heading out next weekend where it will most likely be in the 30's and thought I might add one to my gear. I usually take a high thread count sheet or flannel sheet, depending on the weather. Wondered if I needed to sew 'em up. I have read that a flannel liner can add 12 degrees, a 300 thread count liner can add about 7. So is it worth it, or would good clothing choices be just as good? I'm thinking silk/wool longjohns.