When the KP (KettlePizza) unit was first released, it was individualized to fit each size kettle … 18" and 22", as far as I know.
Later, it was then released to fit the 22", with holes that would adapt to fit the 18", as well.
It will also fit the 26" kettle, but an extra piece needs to be purchased.
The opening / slot to slide the pizza into does not change much from an 18" all the way to a 26" kettle. It's still the same KP unit.
So with all that said …… Ideally, I personally believe that the KP is best suited for a 22" kettle. YMMV
A few members here have had the European (toilet seat) pizza unit …. Not really sure how well it performs.