I made one by encapsulating 2 inches of 4 lb urethane foam (with edges beveled to match the original) in epoxy with a flange around the edge (2 sections like this) and the cover was made out of clear fir plank that was dowelled and edge glued together. This was then cut in half, finished and a piano hinge installed.
The flange on the insulated parts was then drilled and mounted to the bottom of the wooden cover.