I used the back of a stoneware plate taken out of the dishwasher after the drying cycle was done. It was quite warm and stayed that way for a long time and did the job fine. Based on previous battery replacement projects, I thought the back of the phone would have to be pried out of attachment fittings on the phone body, but in fact it was mostly held on with glue, so heating and alcohol along with suction as described on this site are the way to go. Also you should purchase the OEM double sticky tape to reassemble the device nicely after the battery replacement.