I've been looking for these a while, haven't found anything.
TD, this is essentially a seat bottom you'd have hinged and attached somewhere handy. It stays out of the way until you need an extra jump seat, then you fold the seat down and sit down.
Obviously the back side or hinges have to be mounted solidly on something capable of holding a person's weight, which is why I think there aren't many around... not much of a market for them as compared to the pedestal types.
I've come to the conclusion that I'll have to make my own for my own particular need.
cobia and many others, made them years ago, usually mounted on the front of the splash well stern area with piano hinges. they would drop down out of the way. had an aluminum rod, that held them up to sit on.
sorry, i have seen one i years, all the were, were 3/4 plywood base, 2 inches of foam, covered in vinyl, with a 1 1/2 inch piano hinge. a piece of aluminum pipe, with a cane tip on it.