If the deck
has a lot of camber, I'd do the same as Roy did, except out of Coosa board.
If the opening is on a flat spot of deck, and you don't mind a little learning
curve, you can build and shape a mold
in place to lay it up out of fiberglass
without having to do much grinding work at all...
I use adhesive
back sheet wax: FMSC - PS165 Regular Sheet Wax 12" x 24" / 305mm x 610mm (Adhesive back)
Grind a bevel around the existing opening, then put the grinder in a locked cabinet.
Build a platform under the existing opening that is flat. Stack the wax up until you've got the height you want the flange to be, and trim to fit the hatch.
Basically, take hatch, smush into wax, mark the line over a 1/4 so you have room for the fiberglass
you will be laying up into place.
Once you are pleased with the fit, put mylar packing tape over the wax to improve the finish.
Lay up a few layers of cloth, say 6 layers of 17 ounce biax... and a layer of mat, and finish cloth.
To do the layup
, if you don't mind wasting materials in pursuit of an easier completed job with better finish... Cut out full width sheets
of glass, and wet it out 3-4 inches into the waste. No laps, no seams, and a consistent thickness the whole way around. You'll use 2-3 yards of cloth and a bit more resin than you would otherwise, but you won't have to pick up a grinder.
After the layup
is wet out, take a piece of melamine faced MDF that is 1 to 2 inches thick and wax it. Make sure it extends just to the finished edge of the cloth, and use a square edge the whole way around, beveled if it needs to be to follow the contour so you can squeegee up the excess. Leave a finger fillet if you want...
Butter up the cloth face with thickened epoxy, or gelcoat
if you used polyester. Screw it down to the table you made to hold the wax. You can either free-float it if you are good with layups and don't have any laps, only butted seams... Or you can use set-up blocks in between, to hold the top mold evenly spaced. Dry fit it with the cloth in place, so you get your thickness right.
Where there is a low spot after layup, won't be in contact... Where it is good, will be. More thickened epoxy or thickened gelcoat
, the smoother the surface. Try to get good "squeeze out."
When it kicks, destroy the form. You'll end up with a slick shiny inside finish from the mylar tape, and a flat surface on the top that requires very little sanding
because you made a female mold for the top surface with the MDF backer.
Sand and fill only the low spots, first, using the good surrounding area as a guide. Overfill slightly and use a hard wood block 16 inches long, with adhesive backed air inline paper to sand... Take the shine off with 80-120 grit, and lightly finger sand the slick surfaces and pull the entire opening with a finishing putty... If polyester, gelcoat it again. Sand out smooth.
It is a lot less work, if you don't do a raised surface, at the flange, as that doesn't require a finger fillet to blend between the cabintop and the surrounding surface, and can keep all the glass work directly below the flange.
Just don't touch it with 40 grit, or put a grinder on it and the work will go fast. Take the nibs and sharp points off, and then bring the lows up to level and paint
If you want a down-turned flange inside, I would use polyester and mat. Lay up the flange, below the level you want it to be... starting lower than the ground in bevel to the existing deck. Lay that area up as a flat section over the top. I favor using finish cloth to even out the surface texture of the mat, even if it means using a layer of veil mat... Then use a 6 ounce cloth off 2-4 inch rolls and very delicately air roll it out with a 1/4 inch air roller.
Getting mat a consistent thickness is tougher than cloth, but cloth doesn't like to torture into place. Tear the mat, and put reinforcement across the corners on the vertical face and leave the top surface without any laps or bumps.
If you need to make the opening slightly large, a guide for a router works well. Cut opening wider, throw away router bit...
Back yard, press molding...