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If all you want/need is a thwart board seat to use with no backrest, then that is easy enough to make. As I recently wrote about this type of seating, I have cut a 60 inch length of board (I prefer solid mahogany) that is 5/4 inch thick, 11-15 inches width, and place it in the thwart channel. I make sure the board is cut precisely to "friction fit" in the channel, and I do not screw it down. I glue a piece of rubber to the bottom of the board at the ends to help keep it from sliding. I can place such a thwart board seat in front of the console on a Montauk 17 (with cooler removed) or behind the reverse pilot seat, or all the way to the motor well area, where the board will really tighten its fit because of a slight reduction in length between the interior bulkhead channels (beam).
The measurement on a BW16 hull is slightly less in length, at 59 15/16 inches, because the BW16 (no "smirk" hull) is a very different hull.