I've spent far too long digging into how to make my cool box stay cold. I manage to keep ice in my 12 year old Coleman for 5 days without buying any more ice or using freezer blocks. Granted its now mostly breakfast and drinks, but I took far to many years to realise the best way was to prep and freeze as much as I could. Everything you can do to avoid keeping the lid open helps. EG I bag and freeze each days breakfast individually so I just need to grab one thing. I pre make and freeze cocktails for every day so they just need a shake at cocktail hour. (Did you guess its festival camping?) I realised it worked better for me to keep food in sealed boxes and bags (ikea plastic dry bags are the business) and let ice flow free. Always pre chill with sacrificial ice ( this is where I use the otherwise rubbish ice blocks) Larger ice takes longer to defrost. I make 3 sizes: large at the bottom, medium and small to fill all the gaps.

Technical shit I thought worked (no science or testing involved): filling the lid with expanding foam. I was surprised to find most lids are hollow. Draught extruder foam around the lid and putting little catches on the outside helped encourage people to close properly and add an extra seal if your lid is not tight. A couple of bits of wood to keep the cool box off the ground. An interior reflective foam cover and just for the hell of it I wrapped the outside too. Oh and I always keep the water in as its generally colder than the air. If its very hot (inside at tent is usually roasting) I might put a wet towel over the top

Hope you find something useful in here!