I received these seat cushions with a Red / White B Model Penguin. I think that all of 3 styles of cushions were off the shelf items originally made for go-karts. I have not located a source of replacements for any of them.
I received two sets of these seat cushions with Yellow D Model Penguins, they also appear to be the ones used in the Royal Penguin.
These seat cushions were with the 1968 Beaver when I purchased it.