I installed a sliding hatch cover, or sea hood, in New Guinea in 1984. Its simply made of hardwood planks in a flat topped box shape. It used to be varnished but is now painted and has nonskid on top. I would make the next one even simpler from plywood sealed with epoxy and painted and screwed down from inside the cabin. I prefer a flat top so items that are set there do not roll off. A flat top also makes it easy to fit the dodger and the metal track for the dodger to slide into.