There are heating pads for pets (cats and dogs) that keep the warmth very low, near body temperature. You can wash the covers. You would want to put it on just one side of the cage so she could get away from it if she wants. Places like Petsmart and Petco stock them I would call one of those places and ask if they would be suitable for a guinea pig (I think they would). You could make extra covers easily, being a quilter. these are not the same as regular heating pads in that the temperature is constant and mild.