My oldest son and his girlfriend

How do those cupholders attach? Mine have a piece of hard plastic that goes down in-between the seat cushion and the backrest.

If you look at the one you see it has a grey flap attached to it. That goes down between the seats similar to the hard piece that are on the ones that came with the boat. We tend to use these more because you can put them anywhere on the boat not just in the seat. I got them from pontoonstuff.com but don't see them on the site any more.