try a heating pad to soothe the effected side.
If you have one available, place a heating pad under the rags or the box.
These methods include gentle massaging of the shoulder and neck muscles with the fingertips, applying a heating pad to the same area and taking a hot shower or bath.
Just as a warm heating pad helps a human, a dog will be able to benefit from the heat because it relaxes muscles, joints and ligaments.
It's basically an oversized heating pad with a comfortable, protective cover.