It looks like a genuine Kawasaki seat cowl, and it comes with a separate mounting kit that uses the lock that normally holds the rear seat in place.
The guy you brought it from possibly lost the fitting kit.
Try contacting your local Kawasaki dealer and they can source one for you if you don't think the magnet will hold.
Go to your local bike garages/dealers/places they may have the bits there, when I got my bike and seat cowl separate from each other, I didn't have the rubber but that holds the back of the seat cowl down so it was constantly making noises, went to get my exhaust put on and they found the rubbery part to put in win for me
Looks like the rubber grommet is the only thing you are missing. The rubber gromet secures the peg at the rear of the cowel. The trick is that the gromet is only for the cowl, so when you want to put the pillion on you have to pop the gromet out first, hence, they tend to get lost. Not to mention they roll into the weirdest parts of your garage...