My seats are mounted to a swivel fitting that's about 8"x8" (Hamilton Marine). The other side of the fitting is bolted to a piece of 3/4" plywood that's 9"x9". The plywood fits and slides inside the brackets mounted to the top of the countertop. The countertop surface is formica that's glued to .5" marine plywood sealed on both sides with three coats of epoxy, but no fiberglass. The countertop area where the seat base rests spans about 22" front to back and 18" left to right.
I have a CNC machine I use in my carved sign and machining business, so it was nothing to create files to cut all the parts with. The cnc'd parts are the bottom panels, side panels, shelves (gunnels), chine flats, bowstem and transom.