The aft bulkhead of the aft seat/tank is a simple rectangle with softwood seat bearers fixed flush with the top edge. There is a gap between the two seat bearers, where the mast step will go. The seat bearers taper slightly in plan view (ie, narrower on their forward side) to follow the shape of the hull side.

The longitudinal centreline bulkhead in the bow tank is a fairly complex shape with cut-outs at three corners for parts of the mast step and bow transom.

Don't worry that the corner cut-outs of the bulkhead are not exactly the shape of the adjacent parts - any little voids in the corners will be filled with the fillet material when these parts are assembled.

All the dimensions shown for the centreline bulkhead are measured from its aft and top edges. As the bottom of this panel is curved to follow the shape of the bow, the depth of the panel at two intermediate points (200 and 400 from its aft edge) are given.

Mark the four points defining this curve on the piece of ply and bend a batten to fit through all four points - a piece of the 15x15 seat bearer material is ideal for this purpose, but pick a piece that it is fairly straight (not warped) to start with.

A seat bearer is fitted to the top of the centreline bulkhead - you can choose whether to put it to port or starboard.

The seat bearers are 'glued' to the bulkheads with an epoxy/microfibres mix and pinned together while the 'glue' sets.