Again the marking device shown in the build manual came in very handy. Here is it is use.
Experience gained from trimming the cabin roof made this a quick and easy task.
The inboard edges were a little trickier.
There is nowhere to start a saw cut inside the boat, so I drilled a couple of entry holes and used a keyhole saw to remove enough waste to fit in a saw. Like this.
Then, as with the cabin roof, the sander was used in rotary mode with a P40 grit to grind away what waste remained before switching to random orbital mode and P60 and P80 grit to clean up the inboard and outboard edges.
Here are both seatback tops trimmed, seen from astern.