What would you recommend for comfortable bedding/ flooring to avoid bone and joint issues in an outdoor run?

Hi,

I have a 2-and-a-half year old German Shepherd female. I just built her an outdoor chain-link kennel (6X9 feet) with a brushed cement pad floor and a metal roof. She is now in the kennel for about 9 hours a day when I'm away at work.

What would you recommend for comfortable bedding/flooring to avoid bone and joint issues?

Thank you,
Brian