...with a plunge router. I clamp the gunwale board in a workmate and use a simple jig that holds the router fence tight to the side of it. A couple of roller stands provide support where necessary. As long as the mortices are long enough to accept the rib material, it doesn't matter if they're a bit longer. I round the edges of my rib stock to reduce splitting and the possibility of chafing on them, and also to fit the rounded mortices better.