This is pretty out there, but how about taking along something like this:
https://www.amazon.com/Solstice-2935.../dp/B004PPTC9G
Stows easily, comes with a pump that will inflate it quickly, has a 500 lb carrying capacity. The biggest issue will be that it punctures easily, so you'll have to throw a puncture resistant tarp over it. Then, if you can manage to get the fish in there, throw some bags of ice on top of it and then tow it in.