Just a though, cooling the fish down quick..
 We are day fishermen, rather than several days.. thu this has helped when on road trips to middle now wherefor 5/7 days.

I have a couple large thin freezer packs that lay on the floor of the bin.. and can use as a divider between frozen and chilled foods.

Also I have numerous smaller 'fit inside of a gutted 45cm snapper'... and around it.

Gutting fish before putting inside the bin makes them chill down real fast.. less to chill.. longer lasting low temps.

I used to use frozen sea ice in bags, bottles... and normal ice. Then started to add the ice packs...If leave A catch over night, and noticed on roads trips with bagged sea and non sea ice, these always thawed out well after the ice.

 Cold air is heavier so sits in a 'bin' If open more carefully, then slip the fish in.. You retain far more 'chill' if the bin is full.. part full when open and close you loose a lot of air.. full not as much chilled air is lost... Why a full  freezer uses far less power than a 1/2 or 1/4 full one.

So now we dont use any ice or sea ice.. just ice packs, big flat on the bottom, smaller inside and around gutted fish.
On a road trip, after around the 3 or 4th day, and have more space in the bin... we add a bag of ice..still in the bag...
The ice packs still 'rattle' with frozen stuff inside.
 and another 2/3 days later.

Anyway hope this helps, a few ideas ...