I have a snugglesafe heat pad, several UK chicken keepers use them to stand waterers on & are happy with them. I haven’t used mine yet so can’t comment.
https://www.snugglesafe.co.uk/shop/snugglesafe-pet-heatpads/ Link is full of that bigging up their products that manufacturers do

. Apparently there’s a US distributor but I reckon there must be other options there. Cheaper than battery handwarmers but if you have some anyway it may be worth a shot but not sure they’d provide enough heat or last very long