I start with some black pottery or ceramic dishes. They are available at Dollar Tree, WalMart, rummage sales etc.
Park at the jobsite facing the sun. Around coffee break time, I load the dishes with the food from my cooler and put it on the dash.
Then I put the sunshade behind it. By lunch time the food is quite warm, you might need oven mitts to handle the dishes.
Be the envy of your co-workers munching on their cold bologna sandwiches. Put several dishes on the dash and reheat last nights dinner. Things like spaghetti, chili, stew, hot dogs, etc reheat nicely. Be sensible, use it for reheating not cooking raw meats, etc. Common sense says a can of chili in one set of dishes doesn't heat as fast as the same can split into 2 sets of dishes. Experiment. Don't worry about burning food, it is next to impossible with solar cooking. Fortunately I have a black dashboard. This gives back a lot of heat on it's own. You might have to experiment with a black covering on your dash if it is a lighter color. Of course check the weather forcast, without the sun, food will not warm up and spoil.