Do you have a nice shady tree you can move the pool under? Then maybe set up a misting system over him. But put a drain hole so it doesn't fill with water.
The pool is filled with substrate with access to water and food. I use the pool as his habitat but it's too big for the house so I was wondering if he will be okay in the garage during hot days.
I agree with Yvonne that a shady spot would be more suitable than a garage. The hardware store often has those fun little misting attachments for the hose that you can stab into the ground - you could run one of those near the enclosure to cool down the air a bit. Ok, Yvonne already beat me to that piece of advice, too.
You could build a screen top for the enclosure using 1x2s and chicken wire... that would keep any casual wildlife away (maybe not mountain lions, but cats, hawks, etc). The bugs won't hurt.
Chicken wire will work well but you need to make sure that is strongly installed. Because raccoon can easily pry open any unsecured spot. Or you taking him back in the house at night!
Use hardwire cloth, it's heavier and stronger than chicken wire. I would built a frame with 2 x 4's that will fit over your pool and be strong enough to prevent dogs or wildlife from entering the enclosure.