Loop a towel through the kettlebell's handle and perform a standard kettlebell swing holding on to the towel as a handle. Doing it this way brings the forearms into the exercise and will build up your grip immensely. You may want to stand on some wood blocks based on your limb length as you may hit the floor with the kettlebell holding it like this.
Kettlebell Towel Swings Exercise - Start Position
Kettlebell Towel Swings Exercise - Finish Position