PF- you have a good idea there. But first of all don't try to solve everything at once.
The straps, battens or whatever they are on the Trumpeter Liberty ship are in my opinion way too big. Why I think that I don't know because its hard to find pictures of the on deck back then but they scale out to be maybe 9" high and 18" wide.
I'd suspect they were quite a bit thinner and probably less wide.
My Evelyn doesn't have them in any pictures I've seen, and in fact the hatches seem to be open while she's sailing.
Try making the canvas top by itself and adding the straps/ battens with tape or tin foil.
I'm working on how to make the hatch covers look like canvas stretched over wood. Current thinking is nylon stocking material stretched over a basswood shape, immersed in glue and painted.