You'll keep your ice longer if you: 1) have good insulated cooler, 2) have "coldness batteries" such as solid ice blocks that keep the temperature stable, 3) prevent air leaks, which will slowly sap the cold air in the cooler, 4) refrain from opening up the cooler too often, though this is often mitigated by having more ice blocks.
Before you try any "tricks" like keeping your cooler above the ground, covering it in blankets, finding shade, etc. you need to buy a cooler that is actually insulated. All these additional layers of protection are really just bandaids for the typical low-quality coolers with hollow walls. Personally, I only bother with keeping the cooler in shade, away from moving air sources so that it reduces the effects of convection.
Elevating the cooler means that you have a shoddy-ass cooler that is conducting a lot through the bottom surface. It may even facilitating the movement of air under and along the sides of the cooler, which could further degrade the insulating capabilities because of convection. If you're going to elevate your cooler, put it on a solid, poor heat-conducting surface such as on wood. Don't create an open hollow space below the cooler, because while you may think that the air is insulating, the fact that it is mobile means that it will convect.
Most of these good insulated coolers (from Igloo, Coleman, Yeti) use insulating foam that is better than styrofoam. The foam also gives the cooler structure, which is nice if you want to sit on them, or bump them around. Such coolers are not too expensive depending on the season. I got a huge 165qt MaxCold Igloo cooler at Costco for like $60-70 if I recall; it has a nice small door on top for grabbing drinks and avoiding having to open up the entire top. We used that as our main food cooler for about 5-6 of us, including all the vegis we had to bring for a dinner. Sure, we bought ice, but that was because we weren't all that diligent about not grabbing drinks as we wanted them, and we were dumping ice blocks into our water dispenser anyway.
Often a single cooler like that can replace all the multiple smaller, poorly insulated coolers that you bring. Plus, the cold air is shared throughout the entire cooler, and you can often compartmentalize the cooler with your ice blocks. They often have handles on the side for dragging them around, and maybe even wheels. If a 165qt cooler is too much, Costco also sells the smaller MaxCold coolers too.
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