What's involved in making new GRP cockpit locker lids?

[Little Dorrit]

I think from memory that the whole structure must be at least 1.5" maybe 2.0" thick as per foreshortened photo below showing surface teak running lengthwise, so a 'core-box' (i.e. internal ribs running widthwise or even a lattice within a perimeter frame bonded to the internal GRP sides, all of 20mm Iroko or oak) should be plenty strong enough.

View attachment 83309

I think this plan sounds eminently sensible and on reflection would be my approach. I'm not sure if there is any advantages of filling with foam. If your teak deck is like mine there is considerably longitudinal strength. Ply coated in Eposeal would be perfectly waterproof and I think structuraly sound. Ps don't forget to take photographs, I'm not far behind having to do the same!