Our local IGA sells these packs for $2 or $3.
You only need one of the sachets to keep your lens warm. Use the knee wrap to hold it in place. You might have to cut a hole in the wrap to access your focus ring.
Or, you can buy a pair of heated USB gloves and remove the little heating pad and power it using a portable USB power supply.
http://www.ebay.com/itm/Laptop-USB-H...item53f3d11dfe