KF's advice is good and what I use.
One year I caught a Laker on Jean. It was mid morning and in the mid 60's. I gutted it, and removed the head. I put it in a gallon zip lock, squeezed out as much air as possible and put in a canvas money bag I got from my bank.

I rolled the excess canvas down and secured it with a small bungee. I wet it well and laid it under my seat out of the sun. I kept it wet down to Sturgeon Lake and all the way to east end and my nights camp.

At some point I realized it was really hot for early May and checked my thermometer. 90+ :o

The laker grilled up perfect that night and stayed cool all day.
I've also done it overnight by putting it under the top flap of food pack that I always hang in a shaded spot.

Cheap/lightweight, I believe its the way to go without a lot of extra effort/worry.