I would suggest trying to lay in more layers of lighter cloth if you use epoxy. No mat, as the resin lacks the styrene to dissolve the binder in the mat. I would also suggest getting in at least one layer of +/-45 cloth in there. Which can be a diagonally cut piece of 0/90 or a piece of biax. I think if it is like mine, 3 or more layers of thinner cloth will be easier to fit in the corners than fewer thicker layers, like 17-24 oz cloth. Plus wet out will be easier. I would hit Merton's website/catalog.
Personally, I think the because people tend to drill into the hatch to mount leaning posts, the plywood initially helps, as it has some structure (versus foam or end grain balsa) but the grain orientation makes it vulnerable to water ingress, significantly more so than balsa. And heavier. But screws don't need backers as often. And compressive strength is good. But I hate how heavy my gas tank hatch is with a ply core. I will likely change it to divinycel this winter.
The deck on my seafari has a pretty thin inner layer of glass. Maybe 1/16" or a touch more under the balsa core. So if you do it right, you don't need lots.
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