You've heard all the horror stories of expensive equipment getting ruined because it melted in a hot vehicle. It's easy to do. You forget you have that $600 lens sitting in a bag behind the seat. Three hours later in a 120 degree car and that lens is warped beyond repair.
Here's a nice simple trick you can use. Purchase a medium sized ice chest from your local department store and use it to store your sensitive equipment in. While designed to keep the cold in the nice side effect is that it helps keep the heat out.
Two or three ice packs on the bottom with a towel folded over will help keep the interior of the chest cool, even in the hottest of days. The other nice thing is that it keeps your equipment out of sight in a more inconspicuous place of hiding. Less chance to temp a thief.
For you film guys, this trick should be familiar. It was used to keep film safe and cool on those hot outings.