Hmm, whenever I have used rice packs, it hold the heat for maybe an hour. The main difference is the rice is losing the heat, where as the heat pack is creating heat. SO I think for short distances, your ok. The only thing would be to not rely on it for long periods.

As for the project, why not a purse or handbag? I love toying with the idea of making my own purse, I just have no skill for acually taking drawing to real life. lol