I like it.. Can’t wait to see how you get to the end of this project..
Nice work - I’m going to be attempting a similar build on my 2005 ES so this is great documentation to have!
I’m guessing you will include a platform of sorts for feet and some sort of rear bumper? Angle a foot rest to press your foot against to keep from sliding off the back. Maybe even a grab handle at the base of the seat.
Might also consider incorporating a set of lights somewhere since two adults will block the existing lights.
Bonus points for a little folding mini bimini (might be fun).