I'm assuming you're working on the Model Shipways kit. I haven't built it, but I can offer some general suggestions.
The first step is to establish exactly what the gunports are supposed to look like. In a ship of that period they may or may not actually be square. The
Fair American, as I recall, has substantial sheer in her maindeck, meaning that some of the guns - especially those in the stern - sit on a slight but noticeable slope. I'd have to look at the original model of the ship to be sure (as you probably know, the kit is based on a model in the Naval Academy Museum - not on contemporary plans), but I suspect the sides of the ports are vertical and the tops and bottoms are parallel to the deck. In geometric terms, the midships ports, and maybe the ones at the bow, probably are squares, but the after ones are rhombuses. That needs to be noted carefully.
Another point. The sheer plan (side view) on the plans inevitably distorts the proportions of the gunports. The sharp curvature of the bow, for instance, makes the foremost ports look considerably skinnier in the side view than they actually are. There are exceptions to every rule, of course, but in general all gunports for a given size of gun should be of identical width. The only reliable guide to the width of the gunports, though, is the deck plan.
Maybe the trickiest part of this job is to draw the actual shape of each gunport, precisely and accurately, on the surface of the planking. Once that's done, I can't suggest much beyond the careful use of drill and files - and frequent checking.
I doubt that this has helped much. Maybe some other Forum member who's built the kit can do better than I can.
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