don't have much experience with grip stuff, but with fighters, i'll have them place 10lb plates (with more weight hooked on to it) between their fingers and have them do 'farmers walks' while squeezing the plates. or do farmers walks while pinching the smooth sides of two plates together with their thumb and 1-2 fingers... this is mostly for the muscles in the hands though.
there's strongman and grip strength specific forums for this stuff.
I think goheavy.com used to have a grip forum.. been probably 10yrs since I was there though. dunno if it's still up.
I'm sur you can also find stuff on youtube... guys doing pinch grip pullups on 2x4's in ceiling rafters and whatnot.