If you are going to use a cooler to store your food in, line the inside of the lid with Reflectix insulation (silver bubble wrap stuff) you can get at Lowes/Home Depot in the insulation aisle. Makes a big difference.
 
BTW, we don’t serve leftovers to our customers!  WE frown on it! We also take our cart “home” and did so at the only multiday event we have set up at. Our “home” is on wheels (RV) so we plan ahead to be able to put the cart next to us.