Steve,
I have found that in temperatures up to 80 degrees, dressed lake trout conserve surprisingly well if basic measures are taken.
When at all possible, I will place the trout in a colder feeder stream going to, or running near the lake for a minimum of 45 minutes. I have them wrapped in some sort of insulating material during this time. Leaving them in their cool, wrapped cloth/wool bag, I place them in my backpack, which has ideally been in the cooler shade for a few hours.
I have packed fish out for over 5 hours in 70-80 degree temps, and consistently have found that they keep without any issue. No discoloration of the meat, no foul smell, nothing.
I hope this helps. Tight lines