I'll bet my butt that the local lizards use the many many abandoned rodent holes at the hottest parts of the day to get underground far enough so it's cool there. I bet a hole only needs to be 16 inches deep do get dramatically cooler temps.
Some of the lizards living in the Sahara Desert burrow down to get away from 125F surface heat.
If I could, I'd drill a 3 inch diameter hole through my concrete floor where the habitat is. I see I would cut it at 45 degrees, and down far enough into the soil/gravel to get good escape temps. I have the tools, but it's not my house.