Don't put anything in the cage that is electrically powered! She will chew on it. No doubt about it. I know they make electric stick-on heating pads that can go under a cage, on the outside.
Also, I would be hesitant to put any rice filled items in a guinea pigs cage...if she chews thru the outer fabric and has access to the rice itself, it might do her some harm. Their digestive systems were not built for rice.