I've insulated the area under the front hatch on my 13" Revolution and my 13" Ocean Kayak Trident. I used 5/8" closed cell foam sleeping pads, pool noodles, and other closed cell foam for the insulation.
When I'm going to keep a fish, as soon as I catch it I bleed it and put it on ice cubes. I strap a cooler behind my seat for the smaller fish, and insulate the area under the hatch for fish longer than about 30". I put a big huge plastic bag containing crushed ice inside the hatch so I don't gook up the foam and the inside of my kayak with fish slime. I push the bag way up into the nose so a big fish will slide into it without getting tangled. If the bag leaks, I hose out the inside of the kayak and leave all the hatches open until the inside of the kayak dries out.
The insulated hatch does a good job of keeping my fish cold and in good condition during a full day on the water. My Trident has a black hatch cover, so I lay a white towel over the hatch and under the straps which hold the hatch closed, and keep the towel wet. When I'm spearfishing, it's easy to slide forward in the kayaks to load fish in the hatch. When I'm rod fishing it's harder to load fish in the hatches when I'm on the water because my rod holders, fish finder, and other gear interferes with getting access to the hatch. The pedals in my Revolution also interfere with getting fish into the front hatch when I'm out on the water.