Starting to fit the main fore hatch plinth a long job, however, it is worth getting correct

Marking off the fore and aft curves first, then grind away the shape and then spokeshave  down to the lines 

Then start on the fore and aft sides of the plinth and put a bevel on these to lower the plinth on to the deck

a tied fit between the handrails, however enough of a gap to not have a water tarp

Marking round the plinth each time it is shaped until it fits all the way round