Hmm, I think it would work okay but it might not transmit the heat as well as flannel or something that's a little bit thinner. I made some hand warmers with a canvas material once and they didn't feel as warm as others I had made with flannel or fleece. That could have been my imagination though . I didn't do any scientific tests .
I was googling and several blogs said to make sure not to use polyester fleece because it could melt. My Joanns doesn't have fleece that's 100% cotton so I guess I'll use the flannel since it's 100% cotton and won't melt.