We are looking to create a "battery/phone hand warmer" protective phone case for smartphones. Ideally this protective cover will be made of a durable, heat resistant, and waterproof material. As shown on the attached file, an option is to have the cover use expandable bands that can stretch across 4" - 7" phone screens (not impacting the camera lens). We are also open to engineering an option that includes the heating pad and battery pack on a card holder type of case that is applied on to the back of a pre-existing phone case. The heating pad, dimension included on the attached file, will have a pocket for the heating pad and battery pack. Please advise what an engineer thinks as far as the rate of pay and the time expected to complete this project.