Any sort of exercise that removes the thumbs and metacarpophalangeal joints from the equation, if you can close your hand, lock your grip and hang off of your skeleton you'll only add so much to your grip. There are actual crimping blocks and rolling handles you can attatch to weights to strengthen your grip.

Emil Abrahamsson seems to think that hangboarding is the answer to this problem, he suggests holding a hangboard without lifting your total weight off of the ground on the smallest ledge you can manage, twice a day, every day, to turn your grip into iron. He recently beat a lot of pound for pound grip championship records so I think his training techniques are worth paying attention to.

That being said, climbing itself might be the answer since these elite dudes routinely hang off of the absolute tips of their fingers while lifting their bodies up a wall and even for someone who can deadlift a shitton getting used to lifting your weight on crimps takes months to achieve.

It's also worth saying that you have very few muscles in your hand and grip strength is more a game of strengthening tendons and ligaments, which takes a lot longer than strengthening muscles, which might be why one of the guys with the most world records in grip strength right now is 70+ years old.