Estopa, The puck that can be installed on a swing keel is simply a pad that is epoxied to the keel above the pivot point on each side of the keel. It stops the keel slapping from side to side by eliminating the extra room between the keel and the keel housing.
Catalina Direct sells them fairly cheaply or they can be easly home made and epoxied. Look on their site and you can find pictures. It can be done on the trailer.