Originally Posted by Flatswing
why do you need plywood? most corner back cushions are simply a stiff foam (backing) with softer foam on the seat side if that's your preference.
After another look at the original post, I see he has the teak plywood in place to give shape to whatever cushions are added. The rail above curves around the corner, so the plywood back section below must curve around as well. The OP must be asking only about building the cushions for the corner. His question suggests that he will use plywood inside the cushion to get the curve.
My boat has a similar settee, with a curved corner. Our upholstery has been replaced twice. Building the corner cushion was indeed a challenge. It was accomplished by sewing a curve in the edge pieces of fabric and bending a flat piece of 4" foam into the bent cushion after it was sewn, where the zipper was in the bottom edge. It works. You just need to get the dimensions figured out first.
Measure twice, then cut.