In the winter, use you rice/wheat filled socks to keep you warm.. Heat in the microwave for 3 minutes.. They will stay warm for hours.. My wife has made many of these for me. Wheat seems to work the best. Put them in a playmate cooler and take them to the woods with you .. They sure are nice on a zub zero morning. However, they do have an odor that some wildlife might not appreciate .
If you can get some used I.V. bags, fill them moderately full with fine sand and jam a stick in the tubing outlet. Can be left on range, and are waterproof. They'll last 4 years or so. Used to be easy to get, now some hospitals have disposal policies that make them hard to snag.