Frank, I built the ice box for Bietzpadlin.
I used 2 inch thick extruded styrene foam and Great stuff canned foam for gluing it all together and filling the voids. Mine is four inches thick and lined with light fiberglass glued to the foam with 3M spray adhesive and painted with white pigmented epoxy. The top was cutout after the whole thing was assembled but before I had glassed it. The top is tapered all around and has a wood topping on the foam. I use jugs of water frozen at home for ice and when it melts for drinking and cooking. I am going to the boat today and when we get back from an over nighter will post some pictures.