all the advice here you are getting is great.

i would think abt losing the ply rings for the recess, they take up valuable side real estate as well as requiring a larger rim width. tho if you wish, use the kerf method if they are already made.
if by any chance there are patterns for and you have already made the cockpit rim, what abt cutting the deck hole 1" wider all around than the ext profile of the recut rim to say 1" wide , many radial pcs of scrap hot gluing the rim flush, gluing(contact) 1inx2in wide foam under, bridging the gap to the coaming and then just glassing the downturn, base and riser in one step in say carbon fibre or multiple layers of yr fav cloth?

that might be real fast way to do it. there would be no reason(use spacers under the radial scraps) why you could not have the coaming rim follow different contours than that of the deck. (usually more true in diff types of yak, tho)

anyway, whatever method you use: foam, plyrings, strips, plycylindrical - using the actual deck to form or build from will ensure that the pces are most likely to fit in the end.