Fitting a hatch in a cambered deck
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You can cast an epoxy levelling shim, upon which to install your hatch.
You''ll have to fashion a dam, which fits the deck camber (base) and has a flat & level top surface.
Mark out an area larger than the hatch frame, and prepare the deck surface for bonding.
Build a thin wood frame, the inside of which fits the outside of your required shim, and lay on deck . Do it again for the inside (shim) measurement.(They will sit on the crown, and above the valley)
In-fill the valley areas with a pliable putty, like "plumber''s wax".
Cast the epoxy shim, within the two frames.