Can I keep a cooler in a hot trunk for a few hours?

Yes, but only if you pack it right. A trunk in summer can hit 140°F, which is above the danger zone for perishable food. A hard-sided rotomolded cooler (like a Yeti) filled with ice gives you 4–6 hours before temps creep past 40°F. A cheap foam cooler lasts less than half that.