I do realize you said you do not have a microwave but I thought I would post this for all to read as it works so very well. My mother suffers from migraine headaches a lot, she fills tube socks (or maybe another tougher materials for dogs) with uncooked long grain rice. Microwave for a few seconds (alternate to find the right length for your microwave) and this will stay warm about thirty min, no chance of electric shock and reusable. The rice conforms to the body so you can make these any shape you want. Maybe put your rice filled heat pad in the oven for a few min? Not sure, maybe it would work.
A wool pad is an idea as wool does really reflect and retain body heat.
I have purchased thick, 100% wool sweaters at thrift stores and made bed covers with zippers, also used wool batting, they love them.