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Put a solar attic fan on your garage. I did and it keeps my garage about 10-15 degrees cooler.
If there are any heaters or other things that produce ozone in your garage, keep your gear as far away as possible.
No sun, I assume. Why keep the stuff in a suitcase to compound heat? Leave the stuff out and pack it in the suitcase the NIGHT before you leave to go wherever. Or set it out in the car in the suitcase if you leave early A.M.