Padding for Under Dog House Cooling Mats?

My dogs stay outside during the day while we are at work and sleep inside at night. I recently purchased them a wood dog house for shelter from the elements when we are not home to let them come inside. We live in the south, in Alabama where it gets very hot and humid during the day, so I also purchased them cooling mats the kind that are activated by their weight however they are not very soft or comfy.

My question is what is the best thing to use to put underneath the mats to cushion them. One of my dogs is 13 yrs old and has arthritis. I know blankets or regular dog beds would harbor mold and fleas and such. Please help.

By Renee from AL