What kind of cooler should I get for my truck bed or trunk?

Get a cooler that fits your truck bed or trunk, not the other way around. Brands like Yeti, Pelican, and RTIC make coolers designed for these spaces—narrower or lower-profile for truck beds, or shorter and wheeled for trunks. Measure your usable space in three dimensions first. Dedicated coolers are nice, but you can make a standard one work with bungee cords.