Here's an idea I ran with last week:
Find some PVC of a width to be sufficiently punishing. Drill a hole in one end, thread this with plastic tubing. Put a loop of small climbing cord through that (the cord in the photo is rated to 500lbs or so). Wrap this all in rubber splicing tape to improve grip. You can even use two different diameter PVCs and make the grips tapered:
(Or, you could just use towels...)
Now, you just need to use them. Try doing repeaters:
3 - 5 sets / 5 reps of 3-10" with weight
5 sec rest between reps, 2' between sets.
Actively engage your grip while hanging.
Hope this helps.