I have a diving buddy moved here from hawaii, he has one of these kayak kill bags made over there that fits in his front hatch. I must say it looks well made like a high end kill bag for sportsfishers. Expensive though.
http://www.gobananaskayaks.com/brows...lers/2,60.html
Personally, I have a 4'x 3' foot piece of relectix insulation that I fold in half and leave in the bottom of the kayak and forget about it. Unlike burlap that holds water. I wrap my fish in it , maybe add the blue ice block from my lunch. It's basically bubble wrap on steriods with a reflective coating on one side, it's thin and does an amazing job insulating.
http://www.amazon.com/Reflectix-BP48...6179432&sr=1-7