You might want to take a look at the modular sleep system that the military uses. You can find them surplus and cheap in a variety of places, even Amazon, and it gives you a lot of options. There's a summer weight patrol bag rated to 40, a intermediate bag rated to 10, and when ypu zip one into the other, they rate at -10. If you include the Gore-tex bivy, -30 and waterproof. I use one here in WV, used the same in California when I lived there, and am very happy with it. Super durable, easy to clean and fits me well. BTW, that temp rating is what the bag will protect your body heat down to, not what you will be toasty warm at. Add 15 degrees for toastiness, so a 20 degree bag will protect you down to 20, but keep you comfy down to about 35. YMMV.

Jeff