Either buy a legend that has them on, or most people make their own out of a reasonably thick piece of wood and attach using fine rope or cable ties every two inches, with the seat across the corner
Sorry to be rude with above .... I'm thinking about them - they are a grand addition to literally any boat with stern pushpit rails .... The ones I used in Tallinn and a pals boat - he did exactly above ... bought suitable ply and cut to suit etc. Made a real nice job of them. They were even removable when not needed / boat moored up etc.