DIY Portable Heat Source for Travel?

So I've made an appointment for my leo to visit the vet and I've been instructed to include a heat source in her travel container like a "beanie" or a "hot hand" but I'm not sure what those are. I don't have anything like that. The appointment is tomorrow. Is there anything I can use that could be lying around my house?

Thanks ><

P.S. I'm also thinking of documenting my visit so that I can show others what it's like to take a leo to the vet? I'm super anxious about going and I thought maybe if I show others what it's like it might help other anxious people make appointments? Would that be helpful?