I use a canvas coin bag I got from my bank.
I've done it for years.
While in military, I saw a Sgt, sleeve his metal canteen in a old green wool sock and soak before marches in hot weather.
A cool drink of water is a real treat after a 20 mile march!
I started doing it with my backpacking water bottles and continued into my paddling trips.
I caught a small LT on Jean, early morning heading for the east end of Sturgeon lk.
I cut it's head off/gutted and put in gallon ziplock then bank sack and secured with Bungee Bob.
By the time I hit Sturgeon it was in 90's, no wind.
I just kept wetting bag and storing under seat out of the sun.
End of day, the fish felt like it had been in a cooler all day.
I think the effect would even be greater on smaller fillets.
IF you haven't tried this method, I think you'd be pleasantly suprised convenient/conveinent it is.
I've also left fillets in wet bag, and hung with food pack overnight. Fresh fish for layover day breakfast is hard to beat.
A little care packaging them will reduce the odor to near zero so I never had a worry/issue with Yogi being attracted to the combination.