Jocko ended up temporarily nailing cross pieces or braces across the gunwales to counteract this pressure. But I wasn't confortable with the idea of pounding nails into the delicate cedar only to remove them later when another option existed. Some videos I've seen document using thick wooden braces with notches cut into them to fit over the gunwales. So, 6 braces were made with some some knotty scraps not useful for much else and notches were cut roughly cut out.
Using knotty scraps for cross braces
Testing some of the braces for a snug fit
Even though I won't be needing these until the spring, I've put them in "safe" place away from the other scraps so I don't accidently lose them when the rib bending time comes.