I have a colman plastic cooler that is supposed to hold ice for 5 days. Since in the summer here 90s are pretty routine, and triple digits generally happen for at least a couple of days, it's hard to keep everything cool!
So, bought a backpackers ensolite foam pad, cut it to fit the bottom, sides and underside of the lid, and contact cemented it in. huge improvement. the foam pad was about 12 bucks...The cooler will hold ice for 5 days in hot, humid weather
1/2" foam. Maybe you could put in two layers..