How to increase grip strength on overhead bar?

At the end of my workout, I use these bars for dead hang to build upper body strength for pull ups. However, instead of reaching upper body failure, I get hand grip failure due to holding the bar for long time. What can I do to improve grip on these bars?

At the end of my workout, I use these bars for dead hang to build upper body strength for pull ups. However, instead of reaching upper body failure, I get hand grip failure due to holding the bar for long time. What can I do to improve grip on these bars?