Fill a sock with raw rice, tie it off, and pop it in the microwave before getting them. It usually lasts about 45 minutes. You can stop at a gas station halfway through to reheat it. don't let the chicks touch the sock for 10 or so minutes after you heat it up. Or you could wrap two extra socks around the first one and the chicks can touch it right away. This method is great for a carrier, and will likely last 60+ minutes if you use one. Bring a thermometer to make sure the carrier is the rightt temperature. If you can get a more regulated heat source As suggested by the other members, do that. If you cannot, the sock should work fine.