Even a window A/C unit won't help, unless the garage is insulated...

You're stuck, either keep em in the house, or A/C your garage. I'm about 450 miles north of you, I've had my A/C units on full blast since late March... In insulated sheds...

When I was in Florida, I had a big barn that I put two attic fans in, with a big open door on the other end. Even with a constant air flow, they would always slow down breeding in the hot summers.