Whenever I get anyting shipped, usually comes with 2 heat packs just taped to the lid of the styrofoam shipping container.

This is what I did one on the bottom and one on the lid. If you can find the foot warmer pads they get the same temperature and are 2x the size.

I am surprised the ambient temperature isn't higher seeing as the temp coming off them is significantly higher than fish/reptile heat packs. This is interesting. I wonder if they would work better than the fish/reptile heat packs then?