Get the hide off and guts out as soon as you can to accelerate the cooling of the carcass. Bison body temp is around 101 degrees F so any temp up to that is cool comparatively. Game bags or pillow cases go a long way to helping cool down in preparation for putting meat in a cooler. Split large chunks of meat to the bone (rear hams and large shoulders) to facilitate cooling. As others have suggested, keep a prechilled cooler of ice well insulated and shaded. A contractors bag helps keep water off meat however, if the meat goes in the cooler still "hot" the plastic will trap that heat and sour your meat. If not using a bag, I have found that a bath towel on top of frozen water bottles help absorb sweat from the bottles and any fluids from the meat. I really like the wooden rack idea.3 rotomolded and two igloo. Need to fit a cow bison. Only need to cool the meat in the field and overnight before getting it to the butcher. Will qtr the animal and debone ribs, neck, etc. Thenafter processing I’ll put dry ice for trip home. Just going to be hot in the field and need to cool the meat down