Can you take some photos and send them to me so I know what this looks like?
My boat (1974) came stock with a plain Igloo camping cooler as part of the Cruise-Pak. As far as I know this was standard well into the H25 production runs. I modified my interior but provided a drawer-like space under the bottom step of the ladder for a cooler; but I have a problem in that the drain fitting, which I installed, sort of leaks coldness down into the bilge and the '5-day' camping cooler barely keeps ice 2 days any more. I have taken to putting a urethane computer-mouse pad over the drain plug, but it gets smelly.
I have resolved on making a fiberglass insert for this space and installing refrigeration in it, as I am aiming for Florida. It will have to hold ice pretty well too. This fiberglass shell has to be made in three parts, because it has to be installed in-place, in order to take advantage of the full available volume and still have room for 3 inches of foam. I'll provide blog entries on this; but it won't fit your boat because of how I modified mine. However I might be able to make a similar one for yours, if you're interested.
You're not likely to find an original in usable condition. If you find any original at all, especially if it's pretty tatty, you can repair it or just send it to me and I'll make it into a reproduceable part. You're surely not the only guy who will ever have this need! But it's up to you.