I usually start put the recess together before stripping the deck, then i cover with tape and a plastic sheet the parts i dont want to stick to the deck and go on with it, when the time comes i just use a router to cut into the deck and shape the coaming lip. I do the same for the hatches. Of course you have the deck already up, but you can still route your coaming.
Birch ply, the thin "aircraft" one with really thin laminations, like a 3mm 5 plys, its surprisingly strong and if you laminate 2/3 layers of that (Whatever i use it for, i cut off the contours in sections to not waste it so i always have to laminate two layers with staggered joints) it will be plenty. You can go with the top layer against the deck fairly deep laminating the underside so it acts even as reinforcement in front and behind the coaming, just sand down the edges for glassing. Usual glassing would be enough.
I had quite big airplane models crashing and while the rest was reduced to postage stamp sized pieces the parts built with that plywood stayed intact even if they were holed to lighten them. For me its test enough.
The only other quick way to make it its all glass with the foam mould. If you realy worry about weight you can always vacuum bag it on the cheap since its a relatively small thing.