If the part-naming is the same for your SS as it is for my Waterlust, the assembly the cut-out deck hatch rests upon is (from the deck down):

Deck (top layer) / Sill Spacer (to the outside) + Hatch Rim (to the inside) / Hatch Sill (bottom layer).

The water-sealing foam gets stuck to the hatch sill, fits in between sill spacer and hatch rim. Hatch secures on top of this all, same level as deck.

The sill spacer should be narrower than the hatch sill by an amount roughly equal to the width of the seal channel plus the hatch rim. Is it possible you got two hatch sills instead of a spacer and a sill part? Shouldn't be that hard to tell them apart.

(If I had confidence I could post an image here w/o raising my BP 80 pts in the attempt (and likely failing) I'd post a pic a friend sent who's got his Waterlust in the water by now. Picture's worth at least the 176 words I used here....)

The hatch doubler needs to fit within the sill and sill spacer when the hatch is used. Both the sill and spacer should have the same sized/shape piece "left over" from the cnc cutting process- both parts are cut so they'll have a flush fit at the interior edge of the opening, by a cutter making identical paths to trace those two edges.

I built a Shearwater from plans, and didn't use sill spacers (1/4" weatherstripping instead), and in some designs the spacers are made from two C shaped pieces nested on a ply sheet to conserve wood- if that's the case for the SS then these wouldn't have a usable waste piece to use for a doubler.

As long as you're not using the front hatch as the doubler for the back hatch (!), any piece of ply that approximately fits within the sill's inner edge will work. I recall cutting out much of the doubler's middle before gluing, leaving just an 1-1/2" perimeter, to save a little weight...

Looks like they are the same size on have grain veritcal the other horizontal, not sure it matters with plywood

Thanks for all the info