This topic has been discussed a few times in the past but let's refresh ideas here again. I've kept fillets all day or all night during reasonable weather (below 80 in the shade) with a wet canvas bag.
To make this system work to maximum cooling the bag should be generously sized to at least double the size of the amount of fillets inside - a large shopping bag size works great.
Put the fillets in a ziplock with no water then inside the canvas bag. Completely saturate the canvas and hang it in the shade AND where there is as much breeze as possible. The more evaporation the more the cooling effect. This will mean regularly re-wetting the canvas. Do not let it dry out.
If you are not going to be able to tend the bag for awhile you can rig up a large zip lock filled with water hung above the canvas bag with a tiny hole in it so that it continuously leaks onto the canvas bag to keep it wet. That way you can sleep at night knowing your fillets will be kept cool till breakfast.

Remember, the keys to keeping fillets as fresh as possible are wet canvas, shade and breeze = evaporation. The greater the evaporation rate the greater the cooling.

When travelling keep wetting the canvas as needed and store it in a shady spot in the canoe. The stern bilge with a cover for shade usually works well.

Clean your fish quickly before they die on a stringer and store the fillets as above and the quality of the flesh will be much better than keeping them on a stringer for a long time or until they die or become discolored.