Well.. for the past 50 years, I've taken a hot rock from the firepit to bed with me. I just wrap it up in an old cloth, like a piece of old towel.... be sure not to let it get TOO hot. It's VERY NICE and if you cuddle it to your abdomen, warms you up quickly.
Just plan ahead.. while you're sitting around the fire. . . find a nice, clean, smooth rock and put it near the fire, turn it every now and then so it heats all the way through. It'll stay warm until morning!