Box on my old Morgan was awful. Insulation was vermiculite pellets that had gotten soggy and settled between the fiberglass liner and the plywood cabinetwork. Put ice in the box and you'd get condensation on the surface in the cabin!
Worst part of the job was getting the old liner out - seems Charlie had his crew trained on one layup schedule. They used it for the hull and the liner of the reefer! Shoveled out most of the old soggy vermiculite, and vacuumed the rest. Installed styrofoam blueboard insulation, about 3", then laid in a fiberglass/epoxy liner. 24 oz bi-ply in the bottom, lighter stuff on the sides. GREAT improvement.
Also replaced the lid on the box in my Tartan 30. Original was just 3/4" plywood covered with formica. Heavy, cumbersome (where to put it?), and poor insulation. It was two pieces, but both were heavy. Made up a new lid from 2" of blue board with epoxy skin, one piece hinged on the outboard side with a hatch spring to hold it open. More better!