My 2024 Sea2Sky came without the backrest installed, so I installed the backrest after finding the installation instructions online.
I was able to find the metal plug for the hole in the seat.
It was attached to the seat in an odd way. I've attached a couple of pictures.
View attachment 255001
View attachment 255002
The slot on the back had a loop of material through it that looked like it might have been a scrap of the vinyl seat covering. The loop wasn't very long, and was stapled to the underside of the seat to hold the plug in place. So you'd have to devise a method to hold it in place.
And yes, it still has the clear protective film on it, so you'd have to peal that off.
And yes, if you want it I can send it to you. I'm in Tennessee.