It was time to do a proper test fit of the seatbacks, to make sure they were ready to install when required.
First, now that the locker openings had been cut out, I made a quick fit to check that they look OK. They did.
Next I marked out where the temporary screws would be required to hold the seatbacks in place, and drilled all the holes.
I used 3.5mm screws, which are the right size for use on the relatively skinny stringers. I assembled a box of screws with plywood washers, and got out the drill driver.
Here is the starboard seatback, screwed in place.
I didn't use screws along the bottom edge. The screws driven into the seatback support frames hold the bottom in place very nicely, and a heavy weight closes any small gaps.
I'm happy with that.
The port seatback was a good fit too, as here.