I would suggest 2 identical hides, one on the warm and one on the cool side, small enough for the ball to feel the sides of the hide on the sides of his coils when he is inside of them. Also, the tank is really tall and you're losing a lot of the humidity from that (and the glass...) Can you get a shorter tank? Some people use 20Long tanks and say it helps somewhat with the temp and humidity fluxes.
What are you using for belly heat and how are you controlling that heat?
Is the lamp on to keep ambient temps up? Do you keep it on the time?
It's cool that you're reading and figuring it all out!! Keep it up!!