[Rob. Smith]

Make a mold over a rubber or plastic hose ( glue or tack the hose to a sheet of melamine coated ply) in the shape of the opening. Then lay fibreglass over the hose with about a 3/4" lip either side. Make it the correct thickness then lift it off the ply...fibreglass doesn't stick too well to melamine...you will have a nice gutter with a lip on each side...trim it to suit and glass it in. { the inside lip could become the lid "seat" } If you pack the inside lip up a bit ( on the mold ) with plasticine or slicked up cardboard...that would allow some space for a pinch-weld seal.....just a suggestion.