How can I keep water hot?

It needs to be in a soup can and I already wrapped the can in aluminum foil, created a lid made of aluminum foil, fleece and cotton, and put it in a warmer made of two layers of fleece and a stuffing called betting. I put boiling water in at 91 degrees C, and after 20 minutes it was down to 76 C. Is there any way I can make it lose 10 degrees or less? Thanks a ton for your help!