I've been boxing for a eleven years now, but no matter what I do, I can't strengthen my hands. Every time I get into a gripping match with someone, I lose, and I can't seem to find any exercises that help me have a stronger grip.
Bottoms-up kettlebell. It will definitely increase grip strength.
For example, one exercise I do is that I start in a squat and swing the kettlebell from the floor to the bottoms-up position as I stand. I also do an exercise where I hold the kettlebell bottoms-up and step up and down onto a platform. A more standard type of exercise is just to carry the upside-down kettlebell across the length of your gym. The important thing is the kettlebell is upside down.
Now, expect to go light on weight. The bottoms-up is an unstable position and you won't be able to execute the movement with the amount of weight you may be accustomed to. This isn't an exercise for maximum strength or power. It is an exercise for your grip, control, stability, and your core. Stuart Mcgill has studied the exercise and concludes that it activates your core more than most other exercises because it requires such fine-tuned control.
Another aspect of this exercise is that it requires a highly tuned mind-body connection. What I mean by this is that in order to hold a kettlebell bottoms-up you must carefully monitor micromovements in your wrist. One wrong micromovement, and the kettlebell flips over. As you fatigue, the difficulty of holding the kettlebell bottoms-up increases exponentially. Try it, and you'll find out what I mean. Some people find this frustrating, but for me it is a challenge to my mental strength.
If you looking for an exercise that works raw power, this isn't it. That's what squats and deadlifts are for. But if you are interested in a more fine-tuned type of movement it is great.
As a final point, when you go to the gym you will probably not see anybody else doing bottoms-up kettlebells. In the last three years I've only seen two other people doing these exercises. So, you can be a sheep and follow the herd or you can try something different.
I've always found that my forearms/wrists/hands grow significantly when I'm deadlifting frequently. I've also found that my forearms/wrists/hands burn when I do straight-bar bicep curls.
Try incorporating more deadlifts and straight-bar bicep curls into your workouts. If you've never deadlifted, it would be worthwhile to have an experienced friend/trainer show you how to do them, or do a lot of research, because you can injure yourself.