The standard seat rails are fitted running fore and aft and there are two pieces each seat.   It's very difficult to find an easy way to fix the seats to the boat to prevent them being stolen and when emptying water from the boat they fall out.  The seats can also slide about in use.

 If the seat rails are fitted across the hull and a notch cut in the seat sides to fit over them then only one piece is required  [half the weight]  and the seat is securely positioned.  More notches enable changes of position and an extra rail can be fitted to place a  seat for solo paddling.

The seats can be easily fixed and removed by making a brass or  S/S angle which is screwed to the outside of the seat side  and screwed to the rail end protruding past the notch.

The rails I made are 12mm x 18mm