Pinch like a vice, hang on slopers like sandpaper on sandpaper. That's the goal. I'm 5'4" and my hand ape index (tip of thumb to tip of pinky stretched out) is 7.5", but I make every bit of it count.
Grease the groove with a grip strengthener
Every time you go to the bathroom, squeeze a pinch trainer. It's called Grease the Groove, where you do quantity of low-intensity reps to keep your muscles and neurons firing. The theory started with Pavel Tsatsouline, father of the kettlebell, and is used in military applications for training. I swear by doing a set of pinch trainers or hangboard hangs each time I go to the bathroom. Heck, throw in a set of push-ups or sit-ups too.
This grip trainer is A+++ compared to those rubber donuts. It's adjustable from 20-90lb.
4x4 Pinch Hangs
We've got two 4"x4" pinches set up at home, but you can also find some decent pinches in the gym or on a system board. Hang for 7s, rest for 3s, repeat ~7 times per set.
SICGrips Gstring. Hang like hangboard workouts, X sec on, Y sec off. These are completely adjustable for angle. Fip and you have a crimp. This is easily one of the most versatile training devices. Do a 10sec hang each time you walk to the bathroom.
...or get these
Take a stick, drill a hole through, and hang a rope. Curl both ways: pull wrists up to lift the weight, working out the extensors, and roll wrists down to lift the weight, working out flexors. The direction that the rope twists around the stick matters, too.
Or, buy one:
On a system board there's usually rows of mirrored pinches that you can do laps on. Find some decent feet and go up left-right and back down till you pump out. It sucks and it works.