Ok... So We have a vet appt. for Wed and it's cold here in MD esp in the mountains. I need to transport in this cold weather. I am SEVERELY allergic to the heating packs ... you know the camping ones ... so I was thinking of one of those rice packs ... I use them for my joints and back ... nothing in them but rice ... they hold heat for a LONG time do you think it would be fine to use that instead?