I realize I'm paranoid but I bought a large cooler that can hold any of my camera bags. When I know I'm going to be leaving my gear in the car in the hot summer sun for hours, I use it.
I don't use any cooling pack in it, obviously, but it keeps the temperature in the 70s or low 80s inside while the inside of my car, even with the windows cracked, will get well over 100 degrees F. Probably overkill but it makes me feel better.
An added benefit: it looks like I have some cold beverages in there and not thousands of dollars of camera gear.
Kim (the male variety) Canon 1DX2 | 1D IV | 16-35 f/4 IS | 24-105 f/4 IS | 100L IS macro | 70-200mm f/2.8L IS II | 100-400Lii | 50 f/1.8 STM | Canon 1.4X III
RRS tripod and monopod | 580EXII | Cinch 1 & Loop 3 Special Edition | Editing Encouraged