I built a steering rig for a plastic kayak some years back, very simple, similar to a rudder crossbar for an aircraft. It was just varnished baltic birch plywood, 10 mm I think, mounted on a slide with a cord back to the seat for adjustment. A bit crude really (back before I started boatbuilding) but solid enough. So long ago I don't recall all the details. The slide ran in a fore-and-aft groove, part of the molded shape of the hull, because the rails weren't suitable on that boat.