I am wondering if a lightweight alternative to the brick might be one of the reusable hand warmers or a couple of the disposable boot warmers that the hunters use to stay warm might work as well. I usually take a small lunch box size insulated cooler with me anyway to hold misc food items in the car along with a thermos of spiced cider. In winter weather taking a break to hike back to my vehicle might be a welcome warm up time but I usually like easy to transport stuff so I don't lose fishing time.