Make up a set of brackets to fit your seat so that the brackets will hang down inside the rails and under them to allow free movement fore and aft, but prevent the seat from falling off. I have made these brackets out of wood on a rectangular caned wooden seat frame which I made so that the sides of the seat were exactly the same width as the slide rails. Do whatever works, but again, be sure to keep the brackets centered side to side, or your seat won't end up where it's supposed to be. To install the seat, unscrew the tail end of one slider and pull it aside at the rear to open up the space between them, and drop the seat into position. Finding extra long mounting hardware may be a problem.
Most people ask how the seat is held or locked into an intermediate position.
Trust me on this, but it's not necessary to have some means of locking the seat - once the paddler's weight is on it, it won't move. You have to lift your weight to adjust the seat.